Numerical simulation of heat transfer during the microwave heating process of a one-dimensional (1-D) magnetite slab subjected to convective, radiative boundary conditions was performed. The governing equations representing the heating process in the slab were discretized using an explicit finite-difference approach, and a computer code was developed to predict the temperature distributions inside the slab. The heat generation from microwave irradiation dominates the initial temperature rise in the heating and the heat radiation heavily affects the temperature distribution, giving rise to a temperature peak in the predicted temperature profile. As heating continues, the temperature peak migrates inward. The microwave power level is crucial to obtain a high temperature increase rate in the initial heating period (i.e. < 60 s for magnetite). Microwave heating at 915 MHz exhibits better heating homogeneity than 2450 MHz due to larger microwave penetration depth. To minimize/avoid temperature non-uniformity during the microwave heating the optimization of the object dimension should be considered.