© 2018 Advances in Science, Technology and Engineering Systems.All right reserved.During microwave heating of materials, the efficiency of microwave heating depends on the materials' dielectric property, shapes and sizes of the material, materials' position inside the applicator, operating frequency, level of input power, specific heat capacity, number and position of waveguide over the applicator, size and geometry of applicator etc. This paper examines the effects of dielectric properties of the clay sample placed in the multimode applicator on the performance of microwave heating. The simulated and experimentally obtained results show that the variation in clay samples dielectric value creates variation in system efficiency inside the microwave applicator. Also placing another dielectric slab over the material to be heated affects the electric field distribution and system efficiency. Placing KESTAMID dielectric slab over the material improved the heating efficiency by 22%. COMSOL Multiphysics software was used to simulate and estimate the electric field distribution over the surface of the clay sample and inside the multimode applicator for different dielectric property clay samples. The simulated and experimentally obtained results are almost matched.