Thermodynamic properties of Cd_(0.25)Zn_(0.75)Se alloy are studied using quasi harmonic model for pressure range of0 GPa–10 GPa and temperature range 0 K–1000 K. The structural optimization is obtained by self-consistent field calculations and full-potential linearized muffin-tin orbital method with GGA + U as an exchange correlation functional where U = 2.3427 eV is Hubbard potential. The effects of temperature and pressure on bulk modulus, Helmholtz free energy,internal energy, entropy, Debye temperature, Grüneisen parameter, thermal expansion coefficient, and heat capacities of the material are observed and discussed. The bulk modulus, Helmholtz free energy, and Debye temperature are found to be decreased on increasing temperature while there is an increasing behavior with rise of the pressure. Whereas the internal energy has increasing trend with the rise in temperature and it almost remains insensitive to pressure. The entropy of the system increases(decreases) with rise of pressure(temperature). Thermodynamic properties of Cd_ (0.25) Zn_ (0.75) Se alloy are studied using quasi harmonic model for pressure range of 0 GPa - 10 GPa and temperature range 0 K - 1000 K. The structural optimization is obtained by self - consistent field calculations and full - potential linearized muffin-tin orbital method with GGA + U as an exchange correlation functional where U = 2.3427 eV is Hubbard potential. The effects of temperature and pressure on bulk modulus, Helmholtz free energy, internal energy, entropy, Debye temperature, Grüneisen parameter, Thermal expansion coefficient, and heat capacities of the material are observed and discussed. The bulk modulus, Helmholtz free energy, and Debye temperatures are found to be decreased on increasing temperature while there is an increasing behavior with rise of the pressure. Whereas the internal energy has increasing trend with the rise in temperature and it almost remains insensitive to pressure. The entropy of the system increases (decreases) with rise of p ressure (temperature).