NOVELTY - Supplying fuel to fuel cell (4), involves feeding a specific amount of a fuel into a fuel cell; determining a specific monitoring period according to a characteristic value measured from the fuel cell when subjected to a load (5); detecting if the load is varying during the specific monitoring period; and selecting to proceed to the steps according the variation of the load.    USE - For supplying fuel including methanol, ethanol, and boron hydride to fuel cell (claimed) such as an electrochemical energy conversion device for converting chemical energy from a fuel directly into electricity and heat, and useful as direct methanol fuel cell (DMFC) for portable electronic appliances e.g. laptop, personal digital assistant (PDA), global positioning system (GPS), and in industrial requirements, such as those of automobile industry and 3C industry.    ADVANTAGE - The fuel supplying method is capable of measuring a characteristic value of a fuel cell when the fuel cell is subjected to a load while basing on the measurement to determine whether the load is varying and how it is varying if so, and thus effectively controlling the timing for supplying fuel to the fuel cell for optimizing the performance of the fuel cell. The method is capable of regulating fuel concentration and changing fuel supply of a fuel cell according to a loading current measured from the fuel cell for enabling the fuel cell to generate power in respond to the variation of the load and thus optimize the performance of the fuel cell. The method effectively controls the fuel supply of the fuel cell with the use of fuel concentration sensor, and thereby, not only the performance of the fuel cell is optimized, but also the manufacturing cost of the fuel cell is reduced. The fuel cell operating system can automatically regulate its fuel supply for providing a minimum power output under dynamic load. The load is switching with the on/off of the notebook, the method not only can satisfy the minimum requirement of power of the load system, but also it can automatically enable toward a maximum power output. The fuel supplying is capable of regulating fuel concentration and changing fuel supply of a fuel cell according to a load measured from the fuel cell automatically for enabling the fuel cell to generate power in respond to the variation of the load and thus optimize the performance of the fuel cell, which is adapted for all industrial requirements.    DETAILED DESCRIPTION - Supplying fuel to fuel cell (4), involves feeding a specific amount of a fuel into a fuel cell; determining a specific monitoring period according to a characteristic value measured from the fuel cell when subjected to a load (5); detecting if the load is varying during the specific monitoring period; and selecting to proceed to the steps according the variation of the load. The characteristic value is selected from current, voltage and/or power measured from the fuel cell. The load is considered to be varying when the variation of a characteristic value exceeds a predetermined threshold value at any time during the specific monitoring period. The load is determined to be increases when a characteristic value changes from low to high, and to be decreases when the characteristic value changes from high to low. The changing of the characteristic value is determined by the characteristic value changes from low to high when a slope obtained from a curve profiling the variation of the characteristic value is a positive value; the characteristic value changes from high to low when the slope obtained from the curve profiling the variation of the characteristic value is a negative value; the characteristic value from high to low is determined by evaluating whether the difference of characteristic values measured before a specific point of time and at the specific point of time is positive; and the characteristic value from low to high is determined by evaluating whether the difference of characteristic values measured before a specific point of time and at the specific point of time is negative. The fuel sufficiency determination step further involves obtaining a first characteristic value of the fuel cell before the end of the specific monitoring period; obtaining a second characteristic value of the fuel cell at the end of the specific monitoring period; and making an evaluation to determine whether the second characteristic value is small than the first characteristic value; and if smaller, the flow proceeds back to step of obtaining the first characteristics value for injecting the specific amount of a fuel into the mixing tank of the fuel cell system.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of the fuel cell system.    Fuel cell (4)    Load (5)    Measurement device (6)    Controller unit (7)    Fuel feed unit (8)    Cathode (40)    Anode (41)