Molecular dynamic simulation, a kind of theoretical method that supplements high temperature and high pressure researches, was performed on MgSiO3 perovskite. The size effects on the properties of the systems, especially some parameters affected greatly by the system sizes, have been seldom discussed before.Two different sizes of cubes were simulated and, especially, the differences of thermal expansivity, compressibility and equations of state were compared. One 8×8×8 cells cube and one 4×4×4 cells cube were simulated from 300 K to 3?000 K and 0.1 GPa to 100 GPa in this study. The result shows that the equilibrium of temperature and energy not only coincides well with the experiment data but also agrees well with other thermal parameters and equations of state. Thermal expansivity of the two sizes in comparison with diamond anvil experiments indicates that the bigger cube coincides with Funamori et al.'s result. The diversities of the bigger cube are less than 1% from the results of high pressure and high temperature experiments.Consequently, simulating a system as big as possible canhelp to get a relatively reasonable molecular dynamic simulation result.