Copper indium gallium diselenide (CIGS : CuIn₁₋xGaxSe₂) is a high efficiency light absorbent, of which physical properties under ambient pressure have been intensively studied for over 20 years. However, only a few researches under extreme conditions have been carried out for their physical property and in-depth analysis compared to the other semiconductors used for solar cell application such as silicon. In this research, structural phase transitions and physical properties of copper indium diselenide (CIS : CuInSe₂) along with the other IB-III-VI₂ compound were studied under high pressure using density functional theory (DFT). Simulations were performed using CASTEP code. Total energy of the structures under high pressures was calculated in order to determine the stable structure at specific thermodynamic conditions