The melt temperature and a special polymeric nucleating agent [acrylonitrile–styrene copolymer (SAN)] were investigated to find an effective way for tailoring the crystalline structures of the water-assisted injection-molded polypropylene (WAIM PP) parts. The results showed that lowering the melt temperature led to the formation of a small amount of β-form crystals in both outer and core layers of the WAIM PP parts. Nevertheless, the melt temperature had little effect on tailoring the crystalline structures of the WAIM PP parts. The addition of a low content (6 wt%) of the SAN was interestingly found to gradually influence the crystalline structures as lowering the melt temperature. WAIM PP/SAN blend parts with high contents of β-form in both outer and core layers (30.7 and 18.4%, respectively), and high contents of transcrystals in the inner layer were molded at relatively low melt temperature (180°C), whereas the SAN had little influence on the crystalline structures at higher melt temperature (230°C). The formation of the transcrystals was ascribed to the in situ fibrillation of the SAN, which was resulted from high shear and cooling rates caused by high-pressure water penetration during WAIM. POLYM. ENG. SCI., 2013. © 2013 Society of Plastics Engineers