In this research, silica nanopowder is surface modified via a facile urethanation reaction to obtain silicon dioxide (SiO2) nanoparticles possessing amine (NH2)-functionalized organic moieties. Poly(4,4′-oxydiphenylene-pyromellitimide) as one of the most widely used polyimides (PIs) is then uniformly loaded by these organically modified nanoparticles (approximately 5% by weight) during its synthesis process at the thermal cyclodehydration stage of the corresponding poly(amic acid) precursor. Amine-functionalized SiO2 nanoparticles (SiO2∼NH2) and the resulting PI-based nanocomposites (PI/SiO2 and PI/SiO2∼NH2) are thoroughly characterized by Fourier transform infrared spectroscopy, diffuse reflectance ultraviolet–visible, X-ray diffraction, and scanning electron microscopy. Thermogravimetric analyses/derivative thermogravimetry thermograms obviously show that about 22% (by weight) of the nano-SiO2 particles is formed by organic moieties. Moreover, the thermostability order is found to be PI/SiO2 > PI/SiO2∼NH2 > neat PI. The nanocomposite PI/SiO2∼NH2 shows a two-step thermal decomposition pattern; the first step is due to the scission of the organic part from the nanoparticles loaded. According to the slope changes of the differential thermal analyses curves, the first-order phase transition could be found at the thermal range of 300–400 ºC. This transition, however, is not so clear for PI/SiO2 and PI/SiO2∼NH2 nanocomposites due to the interaction of nano-SiO2 particles with PI matrix.