NOVELTY - A fuel concentration in direct liquid fuel cell is controlled by providing direct liquid fuel cell having first and second sets of membrane electrode assemblies; injecting anodic liquid fuels with a known, low-limited concentration into the second set of membrane electrode assemblies; injecting anodic liquid fuels with an unknown concentration into the first and second sets of membrane electrode assemblies; and increasing concentration of the anodic liquid fuels.    USE - For controlling fuel concentration in direct liquid fuel cell.    ADVANTAGE - The method is capable of sensing the concentration of anodic liquid fuels in direct liquid fuel cell during electrochemical reactions.    DETAILED DESCRIPTION - Controlling fuel concentration in a direct liquid fuel cell comprises:    (A) providing direct liquid fuel cell having first and second sets of membrane electrode assemblies, the first set providing currents for loading, and the second set functioning as sensor for detecting a concentration of anodic liquid fuels;    (B) injecting anodic liquid fuels with a known, low-limited concentration (2-8 vol.%) into the second set of membrane electrode assemblies such that the second set performs electrochemical reactions and generates first current at a regular output voltage, and then recording a value for the first current after the first current is stable;    (C) injecting anodic liquid fuels with an unknown concentration into the first and second sets of membrane electrode assemblies such that the second set performs electrochemical reactions and generates third current at the regular output voltage, and the anodic liquid fuels with an unknown concentration injected into the second set of membrane electrode assemblies is maintained at the same temperature as the temperature of the anodic liquid fuels with a known, low-limited concentration; and    (D) increasing the concentration of the anodic liquid fuels, in which injecting the concentrated anodic liquid fuels with an unknown concentration into the first and second sets of membrane electrode assemblies, if I3less than or equal toI1+epsilon.    I3=third current; and    epsilon=concentration tolerance.    DESCRIPTION OF DRAWING(S) - The figure is a schematic view of a direct liquid fuel cell system, which performs the method of controlling fuel concentration.