Microstructure and diffusion mechanism of MgO liquid have been investigated by molecular dynamic simulation. Six models have constructed at temperature of 3800 K in the range of pressure 0 to 25 GPa. Model consists of 2000 atoms (1000 Mg and 1000 O atoms) using Lewis-Catlow potential with periodic boundary condition. The microstructural properties of MgO are analyzed through the pair radial distribution functions, the coordination number distribution, angle distribution and bond-length distribution of basic structural units MgO. (x=2,3,4,5,6,7). The diffusion coefficient of Mg and O atoms is also examined through the mean square displacement. The results show that .the structure of liquid MgO consists of structural units MgO. (x=2,3,4,5,6,7). Upon compression, the size of units decreases slightly but the shape is identical, and does not depend on pressure. The short range order depends very slightly on pressure. Conversely, the immediate range order depends strongly on pressure. For dynamic properties, the diffusion coefficient of Mg and O in liquid MgO is similar to each other and decreasing as increasing pressure.