A modified Green’s function instead of the freespace Green’s function is used as the kernel of integral equation. The modified function is derived to satisfy the conditions of a part of boundary

and the integration is then taken only along the remained part of the boundary and the elements of the final matrix equation are obtained by analytical method rather than numerical method. It results in greatly simplifying the computaion and improving the precision

and in most cases it does not need dealing with singularity problems.Numerical results of characteristic impedance of TEM strip lines and cut-off wavelength of ridged waveguides are given.