Abstract: The ultra-heavy oil reservoir in the Fengcheng Oilfield is characterized by high viscosity and strong heterogeneity, which lead to the long circulating preheating cycle in SAGD (steam assisted gravity drainage) development. The micro-fracturing technology can shorten the preheating cycle. Accordingly, it is necessary to understand the dilation mechanism and rules of reservoirs in the process of micro-fracturing. The representative oil sand core samples were taken from the Fengcheng Oilfield for rock mechanics tests, so as to analyze key mechanical parameters and shear and tensile dilation mechanisms. It is shown that the continental oil sand in the Fengcheng Oilfield is unconsolidated and its dilation angles are much smaller than those of the Alberta marine oil sand in Canada. Furthermore, its mechanical parameters and dilation degree are dependent on mud or bitumen content. The shear dilation near the wellbore increases with the decreasing of confining pressure and the increasing of pore pressure, but that far from the wellbore is tiny. The tensile dilation can be realized by either decreasing confining pressure or increasing pore pressure. The oil sand with strong plasticity can develop apparent plasticity strain during the tensile dilation. It is concluded that the key to the successful in-situ micro-fracturing is to increase the pressure uniformly for water injection and to enlarge its sweep areas.