The purpose of the present article is to modify the surface of nano-titanium dioxide (TiO2) and to investigate the reinforcing effect of both unmodified and surface-modified nano-titanium dioxide (TiO2) on the mechanical properties and thermal stability of natural rubber (NR) nanocomposites. Surface of nano-TiO2 is modified by polyethylene glycol (PEG) and polypropylene glycol (PPG). The effective surface modification of nano-TiO2 is evaluated by Fourier transform infrared (FTIR) spectra and field emission scanning electron microscopy (FESEM). The result notifies that the final properties of NR nanocomposites are dramatically improved in the presence of surface-modified nano-TiO2 in comparison to unmodified nano-TiO2. The excellent reinforcing capability of surface-modified nano-TiO2 is due to its better hydrophobicity and uniform dispersion within the NR matrix, as confirmed from morphological analysis. Furthermore, due to its small size PEG is better surface modifier for nano-TiO2 than PPG.