To explore the effect of inner surface treatments on the thermal performance of grooved heat pipes, a series of experiments were carried out in this work. The outer diameter and length of the grooved circular heat pipes used in these experiments were 6 and 190 mm. The wettability of the inner surface of grooved heat pipes were varied by chemical oxidation treatments. Since the contact angle was depending on wettability, the differentials in the contact angle among the evaporation, adiabatic and condensation sections were achieved by controlling the reaction time between the aqueous solution (1 mol/L NaOH and 0.1 mol/L (NH4)S2O8) and the inner surface. The heat pipes were tested in horizontal orientation, and the results showed that proper treatment methods of the inner surface can effectively improve the thermal performance of the grooved heat pipes. More interestingly, it is demonstrated that the formation of the inner surface with a positive contact angle gradient (the values of contact angle become larger) along the evaporation, adiabatic and condensation sections enabled a decrease more than 12% in the thermal resistance and an increase more than 42% in the dry out limit of the heat pipes. (C) 2013 Elsevier Ltd. All rights reserved.