In this paper, the input impedance and efficiency of rectangulr microstip patch antenna located inside a rectangular wavegudie is calculated by using the cavity model and the mode excited by te patch antenna which is modeled as an equivanlent surface magnetic current on the conducting plate. Measured return losses of a rectangular microstrip patch antenna tuned at 5.93 GHz in the free space and inside the retangular waveguide are compared and found to be in good agreement with calculated results.