Method of Moments (MoM) is one of the powerful method for numerical analysis of antennas or scatterers . Recently, many fast MoM for large scale problems has been proposed and various large scattering problems have been analyzed , . In general, there is trade-off between the accuracy of solution and CPU time in the fast MoM. Nevertheless, in practice, the pattern of the large scattering problems can be obtained accurately using the fast MoM because the scattering pattern is obtained by integration of current over an analysis model. On the other hand, it is difficult to obtain current or input impedance of an antenna in large radiation problems accurately by using the fast MoM because the current is evaluated by the weighted residual and it is not necessarily accurate at each point. Because the accuracy of the solution and CPU time of the fast MoM depend on some parameters, parameter optimization is indispensable to keep a balance between those of the fast MoM.