4,4′-Methylene diphenyl diisocyanate (MDI) played a dual role for preparing thermoplastic polyurethanes (TPU)/silica nanocomposites; both as monomer to synthesize MDI-capped poly(tetramethylene oxide) (PTMO) and as surface modifier of nano-SiO2 particles. Untreated and/or MDI-treated SiO2 were homogeneously incorporated into the resulting TPU polymer after extension of the chains by 1,4-butanediol (BDO). To modify silica nanoparticles, MDI was grafted to the surface silanol groups of the nanoparticles by forming urethane links. MDI-treated SiO2 (SiO2-MDI) nanoparticles, as well as TPU=untreated SiO2 (TPU/SiO2) and TPU=MDI-treated SiO2 (TPU=SiO2-MDI) nanocomposites were characterized by FT-IR, UV-vis DRS, XRD, SEM, mechanical, and thermal analyses.