One way of thermal stability methods and ablation improving for polyurethane elastomers to solid propellant rocket insulation, is using of fillers with high ablation property such as silica particles. In this research, first polyurethane-silica composites (16, 25 and 38% by weight) and polyurethane-silica nanocomposites (5, 10, 16 and 25% by weight) were prepared and then interaction, morphology and thermal stability of them have been investigated by Fourier transform infrared spectroscopy (FTIR & ATR), scanning electron microscopy (SEM), thermal gravimetric analysis (TGA) and corrected thermal gravimetry method. Infrared spectra confirmed the formation of urethane groups and SEM images also showed proper distribution of silica particles in the polyurethane matrix. TGA results too revealed that 50% weight loss temprature of the sample, due to function of silica particles as char trap, were increased with increasing of silica amount that these results were approved by using of corrected thermal gravimetry method again. Residual mass amount of the polyurethane composite contain 25 wt% silica was 0.72 wt% while this amount for the nanocomposite with that same of silica particles was resulted to 1.8 wt%.