The microscopic Hamiltonian of a material, which may be ordered or small disordered, is given. The formulae of refractive index and its temperature coefficient are obtained by using semiclassical method, where the perturbation calcu-lation based on the Hamiltonian is up to third order. From the formulae, the influence of the electron-phonon interaction on the ref active index is studied. It is the first time to point out that the temperature coefficient of refractive index introduced by the electron-phonon interaction (excluded the thermal expansion) is a constant at high tempertnre and is proportional to T3 at low temperature. The contributions of the clectron-phonon interaction and the thermal expansion to the temperature coefficient of the efractive index are positive and negative, respectively. A material with large thermal expansion usually shows a negative tempertur? coefficient, such, as NaCl， while a material with small thermal expansion usually shows positive one, such as ZnSe. The zero dispersion wavelengtli increases with a temperature rise for materials whose thermal expansion can be neglected such as fused quartz. The temperature coefficients of refractive index for some ordered materials and disordered materials have been given according to our theoretical calculation. Theoretical results are compared with experimental data, and a good agrement is obtained.