The role of “epitaxy” of nucleating agent (NA) in nucleation mechanism of polymers was studied to formulate the nucleation rate (I) as a function of concentration of NA (CNA) in mixture of polymer and NA and lateral size of a NA crystal (aNA), I∝CNA/aNA. It is proved that the epitaxy of NA controls nucleation mechanism by confirming above formula experimentally by observing nucleation by means of optical microscopy. We also clarified that heterogeneous nucleation is a probabilistic phenomenon by confirming that “induction time of nucleation (τi)” did not depend on CNA. We established a method to obtain reliable I and τi by adding NA which has been a difficult problem in polymer science.