This paper deals with the preparation and characterization of isotactic polypropylene (PP) containing different metal-polyhedral oligomeric silsesquioxanes (M-POSS). Two M-POSS, Ti(IV)- and Al(III)-isobutyl-POSS, defined by a monomeric structure, were selected for dispersion, and particular attention was paid to the assessment of the morphology, crystallization process and thermal behavior of M-POSS/PP composites, using SEM and TEM microscopy, x-ray diffraction (XRD), FTIR and Raman spectroscopy, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and gas chromatography/mass spectrometry. The study of M-POSS/PP thermal degradation showed that Ti-POSS and Al-POSS induce different effects on the thermoxidative behavior of the polypropylene matrix, showing a clear specificity of the metal center on the PP degradation pathway. In particular, Ti-POSS showed a significant stabilization of PP when heating under air, whereas limited effects were observed with Al-POSS. Moreover, Ti-POSS was also found to affect PP crystallization, driving the crystallization process along specific crystallographic directions.