Abstract: An electromagnetic bandgap(EBG)structure with bow-tie unit is optimized by using particle swarm optimization(PSO)algorithm.The standard PSO has shortcomings,such as premature convergence,searching precision lowness,etc. So based on the simulation of natural death process of 5% B-cell in biology clone selection,this paper proposes particles updating algorithm according to theory of chaos and principle of mutation;and then selects the updated particles in terms of simulated annealing method. Simulation results show that the developed PSO is suitable to optimize the EBG structure with bow-tie unit.The pass band ripples of the optimized EBG structure are very small and basically symmetrical in an approximately unchanged stop band and -10 dB bandwidth, and frequency characteristic of the optimized EBG is well.