A simple theoretical method for the determination of equation of state and bulk modulus under the effect of pressure and temperature is investigated. The calculations are performed for MgO and Mg2SiO4 at different temperatures and atmospheric pressure. The results obtained are discussed in the light of recent research on minerals under the effect of high temperatures. A good agreement between theory and experiment is obtained. The method is also discussed to predict the equation of state and bulk modulus at different pressures along the isotherms 300, 1000, 1500 and 2000K for MgO. The results are found to present a good agreement with the studies based on ab initio calculations at high pressures and high temperatures.