In this paper, usage of a dielectric slab on the top and bottom sides, on the top side, and on the bottom side of a clay sample was presented as a means of improving heating efficiency in multimode applicator. In order to accomplish the optimization procedure, we used four different dielectric slab types having different thickness values. The simulated results show that by using additional dielectric slab it is possible to increase the electric field intensity inside the applicator and obtain higher power absorption rates within the sample and high-efficiency designs.