A moment method analysis of a cavity coupled to a rectangular waveguide through a longitudinal slot has been developed. In essence, integral equations are derived from boundary conditions, an equivalence principle and appropriate Green's functions. Galerkin's method is employed to transform the integral equations into a matrix equation that is suitable for numerical computation. The effects of the primary parameters, namely slot length, slot positions and the dimensions of the cavity, on resonant frequency and scattering parameters are presented. The simulated results are in good agreement with those obtained from a FDTD 3D simulator.