In this study a novel experimental procedure for the crude oil desalting process using microwave field is designed and a mathematical model is developed to predict the global coalescence rates and the evolution of the saltwater separation under the influence of the microwave field. The results of the model are validated by experimental data and a good qualitative agreement between the model predictions and experimental results is observed. Using the model, the effects of microwave residence time, volumetric energy density, and amount of fresh water injection have been studied. The results show that the novel microwave mathematical model can be used in designing the microwave oil desalters.