The poly(amidoamine) (PAMAM) dendrimer-grafted silica nanoparticles were utilized as a curing and composite agent for epoxy-terminated polyurethane (PU). For this purpose, surface-hydroxylated silica nanoparticles (SiH) were modified with (3-aminopropyl)triethoxysilane to obtain the core for PAMAM dendrimer synthesized via consecutive addition of methyl acrylate and ethylenediamine. Effect of different chain extenders, pyromellitic dianhydride (PMDA) and 1,4-butanediol (BD), on thermal characteristics of the composites was also studied in detail. Successful incorporation of PMDA and BD chain extenders and also epoxy groups to the PU backbone was confirmed by proton nuclear magnetic resonance analysis. Chemical modification of silica nanoparticles via PAMAM dendrimer in each of the generation steps was studied using Fourier-transform infrared spectroscopy and also their thermal degradation characteristics. The thermogravimetric analysis results proved that increasing content of the PAMAM-modified silica nanoparticles in combination with incorporation of PMDA as the chain extender resulted in higher thermal stabilities of the PU composites. The cured epoxy-modified PU with PMDA chain extender showed higher char residue compared with the cured epoxy-modified PU with BD chain extender (7.1 against 0.8%). X-ray diffraction analysis confirmed amorphous nature of silica and the PU composites. Scanning and transmission electron microscopies were also used for morphological investigation of the pure and PAMAM-modified silica in addition to the PU composite.