Efficient thermal management of electronics is crucial for their reliable performance and lifetime. Direct cooling of nanoscale heat source components may offer the most efficient thermal management solution. Heat pipe, a phase-change-based cooling device, has been widely used for electronic cooling at system level because of high heat flux cooling capability, but it is challenging to realize in nanoscale, primarily due to the limited choices of fluid circulation mechanisms. This paper presents a nanoscale heat pipe (NHP) which uses a surface diffusion to return the condensate liquid via a nano post connecting the condenser with the evaporator. The NHP is made of a Pt nanogap with the connecting post, which is filled with Ar atoms as the working fluid. The heat transfer modes in the NHP include phase change (evaporation and condensation), convection/conduction heat transfer of Ar atoms, and conduction heat transfer of Pt atoms in the post. This study examines the coupled effects of the average surface temperature, surface temperature difference, Ar number density, condensate mobility, and system size on the heat and mass transfer of the NHP using nonequilibrium molecular dynamics simulation. It was found that the heat transfer by the gaseous Ar atoms in the NHP using the surface-diffusion-driven fluid circulation increases by approximately 44% in the reference NHP design as compared to the heat transfer through the gaseous Ar atoms in a nanogap without a post. For a surface temperature difference Delta T = 60 K, the heat flux of the NHP reaches as high as 240.2 MW/m(2) and the thermal resistance is 2.5 x 10(-2) m(2) K/W. The maximum heat transfer through the adsorbed Ar atoms on the Pt post occurs at an intermediate Ar-Pt surface force epsilon = 1.5 kcal/mol. It is also found that the combined heat transfer through both the gaseous and adsorbed Ar atoms outpaces the conduction heat transfer through the Pt post at the length larger than 1000 angstrom. The effects of the thermal conductivity and cross-sectional area of the post on the NHP performance are also discussed. The in-depth discussions on the nanoscale heat and mass transfer of the NHP will provide an insight into the development of next-generation nanoscale thermal management systems. (C) 2018 Elsevier Ltd. All rights reserved.