In this article, the kinetics of crystallization for semi-crystalline polymers has been successfully incorporated into a cavity flow simulation. The flow resistance, in terms of filling pressure, can be significantly underestimated if crystallization is neglected during the calculation. In addition, the filling behavior for nucleated semi-crystalline materials is quite different from that of its virgin counterpart. With this simulation, the crystallinity distribution and degree of crystallinity in the part can be predicted at any instant of processing time, and hence part quality can be predicted accordingly.