The optical and electrical properties of indium sulfide films with different heat treatments are investigated. Indium sulfide films are heat treated in Ar gas in a temperature range of 100–400 °C. Some annealed samples are heat treated at 300 °C with sulfur powder. The indium sulfide films show a band gap of 1.9–2.3 eV, an electrical resistivity in the range of 5.5 × 10⁰–6.0 × 10³ Ωm, and n-type electrical conduction. The resistivity decreases by three orders of magnitude by heat treatment at 300 °C in Ar gas and recovers almost to the initial state by heat treatment at 300 °C with sulfur powder. The Seebeck coefficient and subgap absorption at 1 eV show similar changes and recovery. The experimental results reveal the possible control of the density of states and of the Fermi level position by heat treatment and, hence, the feasibility of carrier control.