When a spatial method of moments(MoM) is used in conjunction with closed-form Greens functions for the analysis of a general microstrip structures of thin or thick substrate, an efficient technique fur the evaluation of MeM off-diagonal matrix elements as well as diagonal elements is proposed. In order to check the validity of the present method, performance is demonstrated for the example of a coaxially-fed microstrip antenna and the present results are compared with the previous results.