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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 Ra higher than 1 or 2 μm. The material of the spacer examined is SiO2, PMMA, PTFE or Al2O3 and their shape is a right cylinder with 10 mm 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 Ra decisively affects the charging, which decreases as Ra increases. Increasing Ra larger than about 2 μm suppresses the charging until a higher applied voltage is reached, thus improving the insulation property.