Abstract: Based on an analysis of the field data, this paper gives a macroscopically possible explanation on the increase, of the vertical conformance in an oil reservoir as the water injection proceeds. It is considered that both the vertical conformance and the water-oil displacement efficiency are volumetric functions. Therefore, both of them increase synchronously during the oil field development process. The characteristics of the rise of production water-oil ratio gives a reflection of the increase of the vertical conformance essentially. The self-organization of the oil reservoir behavoir system itself is the main mechanism leading to the increasing of vertical conformance in water flooding.