Indium (III) chloride and thiourea were used as the precursor solution to prepare indium sulfide (In2S3) thin films using Ultrasonic spray pyrolysis technique. The deposition temperature (Ts) was set within the range of 320°C to 440°C. The effect of post-thermal on the structure and optical properties of thin films at difference Tp was studied. These thin films were characterized using several techniques, where the X-ray diffraction was used to investigate the crystal structure. The EDS studies point out that the deposited thin films are stoichiometric of In2S3 at TSub=380°C. The UV-VIS spectrophotometer was ,used to study optical properties. The optical results show that the maximum transmittance of thin films is about 80 percent at Tsub = 440°C and the band gap ranges from 2.35 to 2.56 eV within the setting range of Tsub. Hence, these thin films can be used to produce the buffer layer for thin films solar cells or photo voltaic heterojunction devices.