Microwave heating has been widely used in the industry to synthesize dielectric materials, in which conventional procedures are slow, expensive and inefficient. In this application, the energy is introduced directly into the volume of the dielectric and as consequence the quality of the process is highly dependent on the uniformity of the electromagnetic field distribution along the material. That is, the non uniformity of the heating is a potential problem with serious consequences. Thermal runaway is the most critical, in materials with temperature dependent dielectric properties. As the material absorbs electromagnetic energy, the temperature increases as does its ability to absorb more energy. To understand this phenomenon, a perceptive of how electromagnetic field propagates and is absorbed by materials is essential, and then we carried out the 3D electromagnetic field simulation in a microwave oven loaded with ceramic samples, changing several parameters, as size, shape and position of the material, magnetron frequency and dielectric properties of the load.