In this paper, a Method of Moments (MoM) model of packaged microstrip circuit is presented, which is described by Mixed Potential Integral-Equation (MPIE). The Greens functions of a horizontal electric dipole for the covered microstrip circuits can be expressed as the closed form by the Discrete Complex Image Method (DCIM). Then the influence of the enclosure side walls is considered by the equivalent image currents and the impendence matrix is modified further. Finally, the Eleftheriades(1996) model is extended to package model and the packaged microstrip network characteristics are extracted. The numerical results show the package effect could make an important difference on the network characteristics of the microstrip circuits.