TRIP steels have an advantage as structural materials in which strength and ductility can be combined through deformation-induced martensitic transformation. However, these properties on grain-size dependency have not been clarified. In this report, we introduce a grain-size dependency into the energy barrier of phase transformation in the phase-field and dislocation-crystal plasticity model proposed by the authors previously, then investigate computationally the effect of grain size on strength and ductility of TRIP steels from the viewpoint of microstructure formation and dislocation accumulation.