The thermal performance of sintered-type heat pipes can be determined from their permeability, capillary pressure, and capillary speed. These characteristics are closely related to the pore structure, which is influenced by the powder used. To investigate the effects of powder shape on the heat dissipation of a heat pipe, gas atomized, water atomized, and electrolytic copper powders were used in this study. The results showed that the gas atomized spherical powder, despite having the lowest porosity, provided the highest permeability and capillary speed and thus the best heat dissipation. The water atomized irregular powder had a smaller permeability, slightly higher capillary speed, and better thermal performance compared to dendritic electrolytic powder. These results suggest that capillary speed is favorable over the permeability for evaluating whether a copper powder is suitable for heat pipe applications or not. The geometrical factor in the Kozeny-Carman permeability equation, which takes into account the effective pore length, pore surface roughness, and tortuosity, could vary from the 250 of the spherical powder to the 3108 of the dendritic powder for compacts with similar permeabilities, showing the effect of powder shape. The processing parameters, compacting pressure and sintering temperature, were also important. Compacts that were loose-powder-sintered at high temperatures showed higher permeability than those using compaction and low temperature sintering due to the differences in the pore surface roughness. These results demonstrate that the thermal performance of heat pipes is closely related to the powder shape and the process used, in addition to the effects of particle size and particle size distribution.