In situ microfibril structure can significantly improve the mechanical properties of incompatible blends. In this work, the in situ microfibrils were constructed in isotactic polypropylene/polylactic acid (iPP/PLA) blends by direct injection molding process, and the effect of viscosity ratio on the morphology was systematically analyzed. The results of scanning electron microscope and rheology show that the viscosity ratio plays a decisive role in the formation of in situ microfibrils. When the viscosity ratio of PLA/iPP is near 1, deformation and microfibrillation of PLA particles can be conducted due to the larger and more uniform distributed PLA domains as well as stronger viscous drag forces. However, irregular cylinders are observed when the viscosity ratio is far lower than 1. The poor deformation ability of PLA particles should be attributed to the much smaller size and weaker viscous drag forces. The well-defined PLA microfibrils are conducive to avoid damage at the interface, and play a significant role in the enhancement of tensile strength and modulus. This work can simplify the traditional preparation process, construct in situ microfibrils directly by using conventional melt processing techniques and provide a new ideal for the high performance of incompatible polymer blends. POLYM. ENG. SCI., 2020. (c) 2020 Society of Plastics Engineers