Porcelain insulator has been slowly replaced by composite insulator in power grid nowadays. However, flashover fault still occurs on this insulator type. Under certain conditions, partial discharges may occur on the insulator and lead to a complete flashover. The flashover can cause the breakdown and damage the whole power system. Therefore, this paper reports on the flashover voltage of 11kV composite insulator under different conditions. They are pollution methods, insulator surface condition, water content, operating orientations and the ambient temperature. It was found that the flashover voltage can be improved by using 0o operating orientation angle of the insulator, at lower temperature, dry condition, less insulator surface roughness and no pollution on the insulator. Therefore, a better understanding of flashover voltage under different pollution conditions may be attained from this work.