The purpose of this study is 10 examine the relation between the surface temperature change at the evaporator section and the change of inner surface condition of the container due to mercury corrosion. Even if the mercury heat pipe is manufactured very carefully, when that is heated for the first time, the evaporator surface temperature becomes very high. But 30−50 hours later, it will decrease to the normal level. In order to examine the cause, three mercury heat pipes were used. One of them was heated for 10 hours and the second one for 52 hours under vapor temperature of 400 °C. But the third one was heated in the vapor temperature range of 400 °C to 600 °C for about 500 hours. As a result of these tests, it is concluded that the time required for improving the wettability of the inner surface at the evaporator section is about 30 hours, and Cr and Ni elements coming off from the material inside play an important part in the improvement of wettability.