MgO has been deposited on r-plane cut sapphire single crystalline substrate at temperatures between 475 degreesC and 900 degreesC. Transmission electron microscopy (TEM) was mainly performed to study the film microstructures and reveal the film growth kinetics. For the deposition temperatures below 875 degreesC, the MgO film and r-plane cut sapphire substrate mainly maintain an orientation relationship, called OR 1, where [100](MgO) //[11 (2) over bar0]Al2O3 and (010)(MgO) //(1 (1) over bar 04)Al2O3. Below 600 degreesC the initial MgO film shows epitaxial island growth; however the epitaxy breaks down for increasing him thickness. The thickness of the epitaxial MgO film increases with deposition temperature. This effect is discussed by considering atomic diffusion and stress release during high temperature film deposition. High quality epitaxial films can only be obtained in the deposition temperature range between 600 degreesC and 875 degreesC. Film deposited at higher temperatures (above 900 degreesC) are polycrystalline with a variable orientation relationships; the dominate one being [112](Mg) //[01 (1) over bar1]Al2O3, (311)(MgO)//(11 (2) over bar0)Al2O3 called OR 2. A surface reconstruction and a reaction between MgO and Al2O3 at this high temperature are suggested to be responsible for the occurrence of these different orientation relationships.