Thin films of Sn10Sb20Se70-XTeX (0≤X≤8) composition were deposited using thermal evaporation technique. As-prepared films were amorphous as studied by X-ray diffraction. Surface morphology studies revealed that films have surface roughness ~2 nm and av. grain size ~ 30 nm. Optical band gap Eg calculated from transmittance and reflectance data showed a sharp decrease for initial substitution of Se with Te upto 2 at%. Further substitution upto 4 at%, lead to a small increase in Eg value and thereafter it marginally decreased for further substitution beyond 4at%. The trend of optical band gap variation with tellurium content has been qualitatively explained using band model given by Kastner. The dc-conductivity measurements showed thermally activated conduction with single activation energy for the measured temperature regime and followed Mayer-Neldel rule. The dc-activation energy has nearly half the value as that of optical band gap that revealed the intrinsic nature of semiconductor. The annealing below glass transition Tg led to decrease in optical band gap as well as dc-activation energy that might be related to increase of disorder in material with annealing.