The pure tungsten metal prepared by chemical vapor deposition (CVD) have distinct grain size on different surfaces. The high temperature oxidation experiment was carried out in dry air and at 800℃. The effect of grain size on CVD tungsten was studied from the growth curve, phase composition and microstructure of the oxide film. The results show that the fine grain tungsten can form oxide film more quickly during the oxidation process, promote the formation of continuous and compact oxide film, and improve the oxidation resistance of tungsten. The effect of grain size on the oxidation behavior of tungsten is positive. As the tungsten in the fine grain part oxidizes to the interior, the grain size of the surface and the top surface are the same, and the oxidation rate of the bottom and top surface tend to be the same. In addition, the oxidation rate of the deposition layer increases obviously due to the influence of the edge effect.