Fluid transport during hydraulic fracturing can be either proppant or asperity dominated. In the absence of proppants, dilatancy of the natural fractures during shear reactivation is required to provide sufficient aperture and hydraulic conductivity. If a hydraulic fracture intersects a natural fracture, the fracturing fluid enters the natural fractures, increasing pore pressure and decreasing the effective normal stress on the fracture plane. If the effective normal stress becomes low enough, the natural fracture may slip and dilate, resulting in increased aperture and hydraulic conductivity. To understand these processes, we conducted direct shear tests on rock fractures with constant normal load and examined the effect of slip and dilatancy of asperities on mechanical aperture. Using 3D coordinates of the surface asperities and measuring shear displacement and dilation during shear testing, the evolution of the mechanical aperture was calculated as a function of slip and normal stress. The calculation suggested that when effective normal stress is low, small slip events along a rough surface can induce dilatancy along the fracture surface that will cause considerable increases in the hydraulic aperture of the fracture.