During heat treatment processes of steel products, transformation plasticity is known to play an important role as it affects the final product quality such as shape and residual stresses. To investigate this well-known phenomenon, a model coupling crystal plasticity with diffusive phase transformation is developed by using a fast Fourier transform (FFT) numerical scheme. Diffusional transformation including a plastically accommodated volume change is considered (i.e the Greenwood-Johnson mechanism). The model is then used to determine the pre-hardening effect on transformation plasticity. It is revealed that pre-hardening results in an anisotropic transformation strain; pre-tension decreases transformation strain and pre-compression increases transformation expansion along the pre-hardened direction. The model is also used to assess the existing analytical model developed by Leblond and Taleb. In this attempt, some features that contribute to the transformation plasticity are discussed. Among those, it is found that plastic deformation in daughter phase is non-negligibly small especially in the end of phase transformation. These analytical solutions predict linear relation between applied stress and transformation plastic strain. This reasoning attributes the linear relation to the solution of equivalent plastic strain increase with transformation by only the transformation expansion effect neglecting the applied stress effect.