A first-principles calculation based on density functional theory (DFT) was performed for studying high-pressure properties of In(P,S). For calculation method, the concentration of impurity sulfur atom (S) was studied at 6.25% into super cell size 2x2x2 of InP, which called In(P,S). Phase stability and mechanical properties of InP and In(P,S) were investigated up to 10 GPa, and compared between zinc blend and NaCl-like structures. The enthalpy change, between dope-S and undope, and elastic constant under pressure were compared. It was found that the enthalpy change decreases when pressure increasing. All of the elastic constants calculated from DFT with GGA-PBE functional satisfy the Born stability criteria that is the requirements for mechanical stability. Bulk modulus and ductility of InP are increased by adding S atom and pressure.