Maleic anhydride-grafted carbon fiber (CF- g-MAH) was prepared by a solid-phase grafting method. The rheological behavior, morphology, mechanical properties, and nonisothermal crystallization behavior of pure poly(ethylene terephthalate) (PET) and PET/CF and PET/CF- g-MAH composites were investigated. The rheological analyses and mechanical tests show that the addition of CF or CF- g-MAH increased the complex viscosity and mechanical properties of PET. The morphology observations confirm that the introduction of the MAH group on the surfaces of the CF enhanced the interactions between the CF and PET, resulting in the fine dispersion of CF- g-MAH in the PET matrix. In addition, the analyses of the nonisothermal crystallization behavior of pure PET and the PET/CF and PET/CF- g-MAH composites show that CF or CF- g-MAH can act as a heterogeneous nucleating agent in PET and accelerate its crystallization. Compared to CF, CF- g-MAH is a more effective nucleator for PET.