Abstract: The stress sensitivity of micro-fractured, fractured and dense cores from low-permeability reservoir was experimentally analyzed. The non-equivalent diameter and circuitous capillary module was used to express the stress sensitivity of single capillary and porous media by calculating elastic deformation amounts of big capillary and small one. The function mechanism on stress sensitivity of low-permeability reservoir was discussed by using the effective percentage of capillary diameter. The research results show that the stress sensitivity for permeability is more important than that for porosity in the low-permeability reservoir. The difference of stress sensitivity between low-permeability reservoir and medium-high permeability reservoir depends on some distinct factors such as microscopic pore structure, solid-fluid interfacial force and start-up pressure gradient, which can reflect the effective percentage of capillary diameter. The quantitative module of stress-sensitivity considering the effective percentage of capillary diameter can explain the difference of stress sensitivity between low-permeability and medium-high permeability reservoirs based on the microscopic mechanism.