A series of silica particles with different size and surface groups were prepared through the sol–gel process of tetraethylorthosilicate, then directly introduced into polyester polyol resins via in situ (IS) polymerization or blending (BL) method and investigated by Fourier transform infrared spectroscopy (FTIR), thermal gravimetric analysis (TGA), viscosity measurement, particle size analyzer and transmittance electron microscope (TEM), respectively. The results showed that polyester segments were chemically bonded onto silica particles for both IS and BL methods, but more polyester segments chemically bonded onto the surfaces of silica particles during IS polymerization than those during BL process, resulting in lower viscosity of nanocomposite resins from IS polymerization than their corresponding resins from BL method. TEM indicated that silica particles could be dispersed into polymer resins during IS polymerization while causing obvious aggregation during BL method.