The kinetics of the nonisothermal crystallization process of polyhydroxybutyrate in polyhydroxybutyrate/kenaf fiber model composites (with 80/20 and 70/30 w/w matrix/kenaf fibers) were investigated with differential scanning calorimetry. An analysis of the data was carried out with the Avrami, Ozawa, and modified Avrami and Ozawa models, as well as the Kissinger approach, for the determination of the crystallization activation energy. The Ozawa model was unsuitable for analyzing the nonisothermal data, whereas the other models described these systems very well. By the analysis of all the relevant parameters, the nucleation activity of the kenaf fibers was confirmed. The activation energies from the Kissinger method were evaluated to be 41.2, 32.6, and 26.3 kJ/mol for the pure polymer resin and 80/20 and 70/30 (w/w) polyhydroxybutyrate/ kenaf fiber composites, respectively. (c) 2006 Wiley Periodicals, Inc.