This study reports on a three dimensional numerical model to simulate the convection of liquids in a rectangular single-mode microwave cavity. Three-dimensional Finite Difference Time Domain (FDTD) explicit method was employed to predict the electromagnetic field strengths and the absorbed power distributions both in the cavity and the liquid by solving transient Maxwell’s equations. Finite Control Volume method with the SIMPLER algorithm was used to obtain the temperature profiles and flow patterns by solving the unsteady Navier-Stokes equations and energy equations. The nonlinear coupling of the volumetric microwave heating source to the energy equation was handled through an iteration process. The results indicate the coupled phenomena of the microwave resonance, absorption of microwave energy, and convection driven flows. In order to illustrate the phenomena an example of microwave heating of containerized corn oil is provided.