This article investigates the effect of modifying the polypropylene (iPP) α-phase nucleating agent 1,3:2,4-bis(3,4-dimethylbenzylidene) sorbitol (DMDBS) with tetrasilanolphenyl silsesquioxane (phPOSS). It has been proven that an increasing amount of silsesquioxane leads to differences in the crystallization behavior. What is more, it has been observed that the nucleation effect that results from the addition of sorbitol derivatives is suppressed by phPOSS activity. To understand the influence of phPOSS addition on the crystallization kinetics of PP/DMDBS/phPOSS composites that have been prepared by melt processing in a twin screw extruder, differential scanning calorimetry, rotational rheometry and Fourier transform infrared spectroscopy are performed. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40131.