According to recent studies on hydrophobic interactions, this work is devoted to investigate the structural origin of hydration force. As a solute is embedded into water, it mainly affects the structure of interfacial water, which is dependent on the geometric shape of the solute. From this study, hydration repulsive force is reasonably ascribed to the effects of surface roughness of solute on hydrophobic interactions. Hydration repulsive force is only expected as the size of surface roughness being less than Rc (critical radius). Additionally, this can be demonstrated by potential of mean force (PMF) calculated using molecular dynamics simulations.