The model WorkSim is a new tool of analysis for the French labor market. The two main novelties of the model are that it simulates the gross flows of workers between different states (unemployment, employment, inactivity) on the basis of the rational decisions of individual heterogeneous agents, and that both firms and individuals are represented. the model is calibrated by an optimization algorithm, in order to reproduce the situation of the french labor market in 2011. In the present paper, we focus on experiments dealing with the suppression of the fixed duration contracts (FDC) and we unfold the opposing consequences of FDC on unemployment .