Abstract: Measurements of skin friction on a flat plate model were conducted in the 0.6m×0.6m subsonic/transonic/supersonic wind tunnel. Oil film interferometry (OFI) and Preston tube technique were applied to obtain the skin friction data at Ma=0.4~0.8. In order to avoid flow separation, an elliptical leading edge was employed. Results indicate that the agreement between OFI and Preston tube is very well. Viscosity of oil film has little influence on the skin friction measurement. When the stream's Mach number or the total pressure changes, the skin friction coefficient decreases along with the increase of the dynamic pressure or the product of Mach number and Reynolds number (Ma·Re). A freckle-like interference pattern was observed at Ma=0.4 and 0.6, and an assumption based on the new pattern was proposed to determine the transition location. According to the oil motion and CFD results, a separation bubble rather than the new pattern appears at Ma=0.8, and it is responsible for the transition of boundary layer.