Abstract: A tri-band Wilkinson Power Divider using particle swarm optimization is presented. The WPD is composed by trisection transmission lines and triisolation resistances，the design equations are obtained with the concept of the evenodd modes and transmission line analysis.Using particle swarm optimization to improve the efficiency and accuracy of the power divider design. Samples of triband WPD with 0~4GHz operating frequencies are presented. The method is verified by the simulation and test results. The results show that WPD on the designed frequencies have a good impedances matching and isolation. It shows that the particle swarm algorithm has a good application in multifrequency divider design.