Anisotropic dilatation during phase transformation in steels can be caused by banded microstructure. As well as the directionality in mechanical properties, the anisotropic transformation strain can result in distortion and dimension change of the products. Hence, mechanical assessment of this phenomenon is of great importance. In this paper, a numerical approach using Crystal Plasticity Fast Fourier Transform (CPFFT) has been developed and used, reproducing the anisotropic transformation stain during heating (experimental results are reported in Part 1). It was confirmed by the numerical simulation that the banded microstructure in microstructure can be one of the most important reasons for the anisotropy of transformation strain.