Abstract: To investigate the starting performance of high-temperature potassium heat pipes with and without wick under gravity conditions, relevant correlations and susceptibility tests were conducted. The optimal filling volume of the heat pipe with wick is 17.5 g in the test. This filling volume can avoid the slow start-up temperature and the low end temperature caused by insufficient filling, and can also avoid intermittent boiling and transition boiling caused by excessive filling. Heat pipes without wick would be in intermittent boiling condition during starting and heating processes. The severity decreases with the filling volume. Its optimal filling volume is 9 g in the test. In the project scale, the influence of inclination on heat transfer performance of heat pipe can be ignored. In the test state, the starting performance of the potassium heat pipe with wick is better than that without wick.