Integral equation method is used to compute three-dimension-structure capacitance in this paper. Since some multi-conductor structures present regular periodic property, the periodic cell is adopted to reduce the solution domain with adding appropriate magnetic and electric walls. The periodic Green?s function in the integral equation method is represented by infinite series with slow convergence, which can be accelerated greatly by Shanks transformation. The numerical examples demonstrated that the proposed method has both high efficiency and good precision in capacitance extraction for 3-D periodic structures.