Isothermal and nonisothermal crystallization of polypropylene (PP)/calcium carbonate (CaCO3) with two particle sizes (250-300 nm and 20-25 nm) were studied by differential scanning calorimeter. Equilibrium melting temperature (T-m(0)) determined from both linear and nonlinear Hoffman-Weeks relations showed that the composites had a lower T-m(0) as a result of the CaCO3 particles destroying the crystal of PR Kinetics of the isothermal and nonisothermal crystallization was described by Avrami equation. For the first time, without the assumption of a constant U* value, Lauritzen-Hoffman parameters (U* and K-g) were evaluated directly by data-fitting method for isothermal crystallization, and by Vyazovkin's method for nonisothermal crystallization. K-g values obtained were similar with and without the assumption of a constant U* and comparable to those in the literature. Both U* and K-g indicate the dual role the CaCO3 particles as nucleating agents to enhance the crystallization and as obstacles to the chain movement to reduce the crystallization. POLYM. ENG. SCI., 49:1855-1864, 2009. (C) 2009 Society of Plastics Engineers