This paper describes a simple and reliable method of improving the surface insulation strength of a spacer used in vacuum. The method is to roughen the spacer surface to an average roughness R-a higher than 1 or 2 mum. The material of the spacer examined is SiO2, PMNU, PTFE or Al2O3 and their shape is a right cylinder with 10 rum in height and 54 mm in diameter. The spacer is subjected to a ramped dc voltage and its surface charging is observed by using an electrostatic probe embedded in the cathode. It has been found that R-a decisively affects the charging, which decreases as R-a increases. Increasing R-a larger than about 2 mum suppresses the charging until a higher applied voltage is reached, thus improving the insulation property.