Abstract: Matrix decomposition by measured equation of invariance (MDMEI) is introduced into the algorithm of analysis and design of planar phased arrays. By suitable selection of metrons (the measured functions in the MEI terminology), the coefficient matrix in the matrix equation is successfully decomposed and highly thinned down, where the sparsity rate is no more than 4 1%. The numerical results of the radiation patterns show that MDMEI results have rather high accuracy except for some individual farthest sidelobes in the H Plane. This matrix decomposition technique will increase significantly the optimization design efficiency of phased array antennas.