This study theoretically investigates wave propagation properties at the interface of air-skin-fat tissues for a microwave frequency of 6.8 GHz. For this proposes, propagation of the microwaves in a biological tissue model of skin-fat are numerically analyzed by using FDTD method for the transverse magnetic (TM) mode of 2D-Maxwell's equations. These analyses are conducted for various point source placements such as none-contacting, embedded to skin and embedded to fat configurations. Electric field intensity and SAR distribution across the air-skin-fat tissue layers are revealed. The energy penetration and absorption degrees of planar skin-fat tissue layers are discussed in a physical origin. The large electrical impedance mismatches between the tissue layers result in reflection of microwaves from the interface of layers. Skin layer exhibits a considerable wave reflection property due to the high reflection coefficient at air-skin and skin-fat interfaces. (C) 2013 Elsevier GmbH. All rights reserved.