Research was carried out on the crystallization, morphology and flame retardance of isotactic polypropylene (iPP) composites with a polyhedral oligomeric silsesquioxane (POSS). Compounds were analyzed both with and without a coupling agent, maleic anhydride polypropylene (MAPP). This agent was used to improve dispersion. It is used an aminopropylisobutyl POSS as modifier. When the crystallization was studied with DSC and X-ray diffraction, small amounts of POSS were revealed in the matrix and a nucleating effect was produced. The modulus value obtained by DMA shows that a higher amount of POSS (10%) is not necessarily beneficial. Thermal stability was analyzed by TGA. Flame retardancy was studied by means of LOI test. The results suggest that composite with better behaviour is the sample with 98% iPP and 2% amino-POSS and coupling with 10% MAPP (over the amount of iPP).