Three kinds of polyhedral oligomeric silsesquioxane (methyl-POSS, cyclohexyl-POSS, and phenyl-POSS) were chosen to adsorb [3-tert-Bu-2-O-C6H3CH=N(C6F5)](2)TiCl2 catalyst (FI catalyst) in order to synthesize ultrahigh molecular weight polyethylene (UHMWPE)/POSS nanocomposites according ethylene in situ polymerization. It was shown that the characteristic of living polymerization of the FI catalyst would be maintained when alkyl-POSS was incorporated. However, the presence of alkyl-POSS could decay the catalyst activity. The incorporation of alkyl-POSS led to an increase of the crystallinity and the lamellar thickness of nascent UHMWPE, as observed by differential scanning calorimetry. This indicated that the alkyl-POSS would be used as a nucleating agent during the chain growth procedure. Interestingly, all the nanocomposites exhibited low starting storage modulus values which was a reflection of disentangled structure in the nascent UHMWPE nanocomposites. However, polymer chains were prior to entanglement with the incorporation of alkyl-POSS. Rheology and crystallization studies were used to discuss the formation mechanism of entangled structure in the synthesized UHMWPE/POSS nanocomposites.