Quantitative analysis of isothermal crystallization kinetics of PLA/clay nanocomposite and PLA/clay/regenerated cellulose fiber (RCF) hybrid composite has been conducted. The crystallization rate constant (k) according to Avrami equation was higher in PLA/clay nanocomposite than in PLA/clay/RCF hybrid composite at the same crystallization temperature. The equilibrium melting temperature obtained by Hoffman-Weeks equation was almost same in both composites, whereas stability parameter was greater in hybrid composite than in nano-composite. Activation energy of hybrid composite for crystallization was larger than that of nanocomposite. The value of nucleation parameter (K-g) and surface free energy (s(e)) of hybrid composite were larger than nanocomposite, indicating that hybrid composite has a less folding regularity than nanocomposite. (c) 2008 Wiley Periodicals, Inc.