NOVELTY - Preparing biodiesel from natural oils and/or fats (I) comprises: mixing and reacting a oil/fat (1), a alcohol (2) and a catalyst (3) and separating a first dense phase (5) and a first light phase (4); mixing and reacting a oil/fat and a alcohol with catalyst, and separating the second dense and light phase; reacting the first and second light phases with alcohol and catalyst, and separating the third dense and light phase; reacting the third light phase with neutralizing agent, and separating the fourth dense and light phase; and followed by purifying.    USE - The invention deals with the process of preparing the biodiesel.    ADVANTAGE - The process: is cost effective; minimizes the moisture content of the recovered alcohol; is carried out by transesterification of natural oils and/or fats and, preferably castor oil, in the presence of a transesterification catalyst; minimizes consumption of energy and the cost of treating aqueous effluents; minimizes catalyst consumption; reduces the consumption of water and acid used for neutralization; recovers excess alcohol, which is used for biodiesel production, with a low water content; and facilitates the washing process.    DETAILED DESCRIPTION - Preparation of biodiesel from natural oils and/or fats in the presence of a low molecular weight alcohol and a catalyst, comprises: directing, under process conditions of temperature from ambient temperature to 140degreesC, pressure from atmospheric pressure to 10 bars and a molar ratio of alcohol/(oil/fat) of 3-30, oil/fat, alcohol and catalyst, to a first reaction module (MR-1A) to mix, react, and separate a first dense phase and a first light phase, formed during the reaction; directing oil/fat and alcohol to a second reaction module (MR-1B), together with a fraction containing residual catalyst from the other reaction modules (MR-1A and MR-2), mixing and reacting the oil or fat, the alcohol and the fraction under reaction conditions preferably similar to those in the first reaction module (MR-1A), and after the required reaction period separating the resulting product into a second dense phase and a second light phase; directing the first and second light phases to a third reaction module (MR-2), together with alcohol and catalyst, reacting them under reaction conditions preferably similar to those of the first reaction module (MR-1A), and after the required reaction period separating the resulting products into a third dense phase and a third light phase; directing the third light phase leaving the third reaction module (MR-2), sent as a phase, to a purification system (SP-1); directing the second dense phase separated in the second reaction module (MR-1B) and the excess, if any, of other dense phases, to a recovery system (SREC-1); directing the third light phase arriving at the purification system (SP-1) to a neutralization system (SN-1) where it reacts with a neutralizing agent, and then separating the resulting product into a fourth dense phase, which is directed to a recovery system (SREC-1) and a fourth light phase, which is directed to a washing system (SLAV-1); removing water-soluble contaminants present in the light phase in the washing system (SLAV-1), with the help of water; and separating a fifth light phase which leaves the washing system (SLAV-1), and directing it to a drying and wax removal system (SSRC-1), to give (I), and preferably directing separated aqueous phase to the recovery system (SREC-1).    DESCRIPTION OF DRAWING(S) - The figure shows the flow diagram of the process.    oil/fat (1)    Alcohol (2)    Catalyst (3)    First light phase (4)    First dense phase (5)