The electromagnetic excitation of a discharge-loaded microwave cavity plasma reactor used for diamond thin film deposition has been numerically modeled in the time domain. This reactor model simulates a three-dimensional, cylindrical, single-mode excited cavity including the input power coupling probe. The time-varying electromagnetic fields inside the resonant cavity, both inside and outside the discharge region, are obtained by applying a finite-difference time-domain method to solve Maxwell’s equations. The boundary conditions and electromagnetic field excitation methods of this model are discussed. The microwave electric field interactions with the plasma discharge are described using a finite-difference solution of the electron momentum transport equation. The spatial electric field patterns, power deposition patterns, stored energy, and quality factor of a cavity loaded with hydrogen discharges at various pressures (20-70 Torr) are simulated and compared with experimental data.