Chalcogenide Se50Te20S30 thin film of different thickness was deposited using thermal evaporation technique. The thermogram of the chalcogenide bulk Se50- Te20S30 was obtained using a differential scanning calorimetry (DSC) with heating rate of 7.5 K/min. The glass transition temperature Tg, crystallization temperature Tc and peak crystallization temperature Tp were identified. The X-ray diffraction (XRD) examination indicates the amorphous nature of the as-deposited film and polycrystalline structure of the thermal annealed ones. The dark electrical resistivity (q) measurements were taken in temperature range (300–500 K) and thickness range (200–450 nm). Analysis of the electrical resistivity results revealed two types of conduction mechanisms: conduction due to extended states in the temperature range (T[Tc) and variable range hopping in the temperature range (TTc). The effect of the heat treatment and thickness on the density of localized states at the Fermi level N(EF) and hopping parameters were studied.