This work concerns a mathematical modeling and a numerical simulation of the transesterification reaction of lipids extracted from micro-algae by biodiesel. We have developed a numerical model to simulate the consumption of reagents, the evolution of reaction intermediates and the formation of products. The model is based on the Euler and Runge-Kutta methods. The system of equation established from the different reactions that take place during the trans-esterification has been solved using a numerical code written in FORTRAN. The simulation gives the evolution of the relative concentrations of reactants, intermediates and products of the reaction as a function of time. The duration of reagent consumption and product formation reactions is shorter when equation resolution is done by the Runge-Kutta method. The results obtained by the latter are close to those of the literature