Understanding of thin film growth mechanism is crucial for tailoring film growth behaviors,which in turn determine physicochemical properties of the resulting films.Here,vapor-growth of tungsten carbide overlayers on W(110) surface is investigated by real time low energy electron microscopy.The surface growth is strongly confined by surface steps,which is in contrast with overlayer growth crossing steps in a so-called carpet-like growth mode for example in graphene growth on metal surfaces.Density functional theory calculations indicate that the step-confined growth is caused by the strong interaction of the forming carbide overlayer with the substrate blocking cross-step growth of the film.Furthermore,the tungsten carbide growth within each terrace is facilitated by the supply of carbon atoms from near-surface regions at high temperatures.These findings suggest the critical role of near-surface atom diffusion and step confinement effects in the thin film growth,which may be active in many film growth systems.