A micromechanical model of a diffusional transformation has been developed, which describes the progress of the transformation within a three-dimensional "unit cell" submitted to an external stress state. The example chosen is that of an isothermal pearlitic transformation of a steel. The transformation plastic strain is due to interactions occurring between the local stresses and the transformation process, resulting in an oriented plastic flow in or in the vicinity of the material layers swept by the transformation front. The ana lysis of the local mechanical states of the simulation provides a good interpretation of the evolution of transformation plastic strain, when considering the effect of applied stress level and the way mechanical properties are imposed on the newly formed phase. In particular, the normality law for transformation plasticity is related to the shape of the local plastic zones. The discrepancy observed between simulation and experience is then discussed, following two main points : the influence of the behaviour law of the phases and the way interactions between neighbouring cells are prescribed. The difficulty and importance of obtaining realistic mechanical properties of the forming pearlite is pointed out.