Unlike conventional reservoirs, tight reservoirs have complex pore structures and severe boundary-layer effect. The pore throat of tight reservoirs is in nanoscale and the boundary layer cannot be ignored because the boundary layer has an important effect to the fluid flow and its influence increases with the reduction in the pore throat radius. These are the main reasons for the ultra-low permeability and low oil recovery for these reservoirs. However, previous studies have paid limited attention to the influences of the boundary-layer effect and the pore size distribution. In this paper, a new model was built to determine the effective permeability for the tight gas and oil reservoirs by taking into account the boundary-layer effect and the pore distribution. The results from this new model show good agreement with the experiment data, and the main factors that impact the effective permeability were analyzed in the study. It is found that the fluid type, means and standard deviation of pore radius, and displacement pressure gradient are the main factors influencing effective permeability. The relationship of air permeability and liquid permeability is also analyzed for tight reservoirs.