Indium phosphide is the only compound in the In-P binary system, and it is a semiconducting material used in optoelectronics, microelectronics, and solar cells. Understanding the thermodynamic properties of this material is important for these applications. Phase diagram of the In-P binary system was determined at 1 bar pressure and temperature range 200-1400 K using Thermo-Calc 2019b Software, and compared with experimental and calculated ones found in the literature. There are four phases, and six fields of which are four mixed (double phase) fields. The melting points of In and P elements, and InP compound were determine and all reactions are determined too.