An experimental study of heat transfer performance in 3D internally finned steel-water heat pipe was carried out in this project. All the main parameters that can significantly influence the heat transfer performance of heat pipe, such as working temperature, heat flux, inclination angle, working fluid fill ratio (defined by the evaporation volume), have been examined. Within the experimental conditions (working temperature 40 degrees C-95 degrees C, heat flux 5.0 kw/m(2)-40 kw/m(2), inclination angle 2-90 degrees), the evaporation and condensation heat transfer coefficients in 3D internally finned heat pipe are found to be increased by 50-100% and 100-200%, respectively, as compared to the smooth gravity-assisted heat pipe under the same conditions. Therefore, it is concluded that the special structures of 3D-fins on the inner wall can significantly reduce the internal thermal resistance of heat pipe and then greatly enhance its heat transfer performance. (c) 2006 Elsevier Ltd. All rights reserved.