A rigorous full-wave analysis is employed to analyze a shielded microstrip T-junction. A method of moments technique is employed on the T-junction apertures of microstrip lines. A multi-step Green's function approach is employed to determine the integral equations related to aperture fields. In this context the Green's function of the T-junction enclosed inside a shielded microstrip is determined by applying semi-analytic techniques. The scattering parameters of the T-junction are computed for various cases and are presented for a wide range of frequencies. The importance of shielding box modes is investigated.