The effect of fiber reinforcement on the crystallization kinetics of poly(ethylene terephthalate), or PET, was investigated using differential scanning calorimetry. The objective of the study was to determine how the effects of fiber reinforcement on PET crystallization are modified by the presence of polymer nucleating and plasticizing additives. Unidirectional fiber composites were prepared using aramid and glass fibers in PET. The rate of crystallization of PET, as reflected by crystallization half-time, is seen to depend on reinforcing fiber type, crystallization temperature, and presence of nucleant or plasticizer. However, degree of crystallinity of PET is largely unaffected by the presence of additives and reinforcing fibers. Crystallization kinetics are analyzed using the Avrami model for PET volume crystallized as a function of time. Avrami plots for PET and fiber-reinforced PET exhibit two linear regions, possibly corresponding to primary and secondary crystallization, The crystalline morphology of fiber-reinforced PET was also studied using polarized light microscopy. Results concerning nucleation density and growth morphology are used in explaining differences seen in crystallization kinetics in fiber reinforced systems.