Three polypropylene-g-polystyrene (PP-g-PS) graft copolymers with the same branch density but different branch lengths were evaluated as compatibilizing agents for PP/PS blends. The morphological and rheological results revealed that the addition of PP-g-PS graft copolymers significantly reduced the PS particle size and enhanced the interfacial adhesion between PP and PS phases. Furthermore, it is verified that the branch length of PP-g-PS graft copolymer had opposite effects on its compatibilizing effect: on one hand, increasing the branch length could improve the compatibilizing effect of graft copolymer on PP/PS blends, demonstrated by the reduction of PS particle size and the enhancement of interfacial adhesion; on the other hand, increasing the branch length would increase the melt viscosity of PP-g-PS graft copolymer, which prevented it from migrating effectively to the interface of blend components. Additionally, the crystallization and melting behaviors of PP and PP/PS blends were compared. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40126.