By adding a polymeric -nucleating agent (acrylonitrile-styrene copolymer, SAN), in situ microfibril reinforced isotactic polypropylene (iPP)/SAN blend parts with high contents of -form crystals and transcrystals were molded via water-assisted injection molding (WAIM). Thanks to the unique stress and temperature fields occurring during the WAIM, SAN microfibers formed across the whole residual wall of iPP/SAN blend parts with relatively large thickness. Numerical simulations on high-pressure water penetration and cooling stages of the WAIM were carried out to reveal the stress and temperature fields. Comprehensive analysis of both experimental and simulated results showed that not only the shear flow field but also elongational flow field occurring during the WAIM was responsible for the formation of SAN microfibers and unique crystal morphology distribution in the WAIM iPP/SAN blend part. Moreover, during the WAIM, the high cooling rate also played an important role in the formation of both phase and crystal morphologies. The preferential formation of transcrystals in the inner layer of WAIM iPP/SAN blend part could be ascribed to the strong elongation, rather than the strong shear. It was believed that the quantification of stress and temperature fields of the WAIM via numerical simulation could provide a guidence for molding high-performance products. POLYM. ENG. SCI., 55:1698-1705, 2015. (c) 2014 Society of Plastics Engineers