Electron heat diffusivity inferred from the electron power balance was studied in the ohmically heated plasmaon an HT-7 tokamak. The results obtained before and after siliconization were compared and it is found that the diffusivity is reduced obviously in the half of the outer region (r/a ＞ 0.5) after siliconization, which was the reason why energy confinement was improved after siliconization.