The aim of this work has been to study the crystallization behavior of composites based on polylactic acid (PLA) and three different types of cellulose reinforcements, viz., microcrystalline cellulose (MCC), cellulose fibers (CFs), and wood flour (WF). The primary interest was to determine how the size, chemical composition, and the surface topography of cellulosic materials affect the crystallization of PLA. The studied composite materials were com pounded using a twin-screw extruder and injection-molded to test samples. The content of cellulose reinforcements were 25% by weight. The MCC and WF were shown to have a better nucleating ability than CFs based on differential scanning calorimetry and optical microscopy studies. It is difficult to visualize that transcrystallization will occur during melting process and this process is influenced by the morphological and chemical characteristics of the reinforcement. Bulk crystallization seems to be mainly dependent on the processing temperature. The cold crystallization process was shown to improve the thermal stability and storage modulus of the composites. (c) 2006 Wiley Periodicals, Inc.