The effect of thermal stresses on the temperature dependence of the refractive index for tantalum pentaoxide (Ta2O5) dielectric films deposited on Si substrate was investigated experimentally. The thermally induced variation of refractive index of Ta2O5 films was characterized by measuring the index-temperature coefficient of the antiresonant reflecting optical waveguide with a Mach-Zehnder interferometry system and a film stress measurement system. The measured results with and without the thermal stresses were 7.89 x 10(-6) and 3.64 x 10(-6)/degrees C at 632.8 nm from 25 to 55 degrees C, respectively. A comparison of the measured results showed that the index-temperature coefficient of the Ta2O5 films with thermally induced stresses was about two times higher than those of films without thermal stresses. This indicates that the effect of thermally induced stresses on the temperature dependence of the refractive index for Ta2O5 films cannot be neglected. In particular, the optical properties of the index-temperature coefficient were characterized in the small range of 10(-6)/degrees C.