Molybdenum carbide has attracted much research attention for its precious metal-like catalytic properties, especially in hydrogen-involved reactions. It possesses rich crystal and surface structures leading to different activity and product selectivity. With advances in nanoengineering and new understanding of their surfaces and interfaces, one can control the transition between different phases and surface structures for molybdenum carbide nanoparticles. In this context, it is essential to understand their surface compositions and structures under operating conditions in addition to their intrinsic ones under ambient conditions without external cues. The necessity of surface study also comes from the mild oxidation brought by passivation in carbide nanoparticles. made using the bottom-up synthesis or solid-gas phase temperature-programmed reduction. In this perspective, we first introduce the relevant crystal structures of molybdenum carbides and highlight the features of the three types of chemical bonding within. We then briefly review the studies of thermodynamically favored surface components and nanostructures for partially oxidized molybdenum carbide nanoparticles based on both experimental and theoretical data. An electrochemical oxidation method is used to illustrate the feasibility in controlling and understanding the surface oxidation. Finally, structure-property relationship is discussed with several recent examples, focusing on the effect of phase dependency on the adsorption energy of reaction intermediates.