Abstract: Different layouts of microphone planar array had different performance of direction finding (PDF). However, according to existing array design methods, they couldn’t optimize the PDF of microphone planar array to reach its optimum but also took high computational complexity. Aimed at this issue, a design method of the optimal microphone symmetrical array based on the Cramer Rao Bound was proposed in this paper. A constraint condition was presented in this method and it could make array element satisfy specific relationship between each other which could improve the PDF of the array. Besides, it also could reduce the amount of computation of the array design. The optimal microphone symmetrical array was designed by this method, and the theoretical analysis and simulation of the PDF about this array were conducted too. By comparing with other common layout arrays, the results indicate that the PDF of this array obtains its optimum.