Functionalized multiwalled carbon nanotubes (FMWCNTs) have been introduced into immiscible poly-styrene/polypropylene (PS/PP) blends. Not only the change in glass-transition temperatures of PS and PP but also the crystallization and melting behaviors of PP have been investigated. The results show that FMWCNTs exhibit dual effects in the blends. One is that FMWCNTs, as a plasticizer for both PS and PP, induce the change in glass-transition behaviors in both PS and PR The other is that FMWCNTs, as a heterogeneous nucleating agent for PP, change the crystallization behavior of PP from homogeneous/bulk crystallization process to only bulk crystallization process. Further results show that the presence of FMWCNTs also induces the morphology change of the immiscible blends to a certain degree. POLYM. ENG. SCI., 51:585-591, 2011. (C) 2011 Society of Plastics Engineers