This paper presents an approach to prioritize maintenance tasks in power distribution system based on reliability models. This approach define weighted average system reliability index WASRI employing reliability indices SAIFI, SAIDI, MAIFI and interruption cost, which reflecting failure cost of specific component. The benefit of maintenance task is defined as improvement of WASRI. This approach ranks maintenance task based on benefit-to-cost index which related with WASRI. Compared with traditional TBM and CBM, this approach could improve the effectiveness of maintenance tasks obviously under same budget limit.