Dental impression materials are necessary for prosthodontic treatment and should have some desirable properties that easy to use, reprocuctibility to oral tissue, dimensional stability and compatibility to gypsum materials and additional silicone impression materials are used mostly. All materials have dimensional charge by thermal changes. So, impression materials shrink during cooing from oral temperature(37℃) to room temperature(23℃). 5 kinds of light body of addition silicone materials(Imprint Ⅱ(IM), 3M Co., USA; Examix(EX), GC Co., Japan; Extrude(EX), Kerr Co., USA; Contrast(CT), Voco Co., Germany; Perfect(PF), Hankook JP, Korea) were chosen and the cylinderical specimens(6 ㎜ dia., 12㎜ height)were made and thermal expansion were measured by thermomechanical analyzer(TMA 2940, TA Instrument, U.S.A.) between 23℃ and 37℃. The data were analyzed by Mann Whitney U-Test. The results were as follows: 1. Thermal expansions of impression materials were increased in order to CT>PF) EM>EX≥IM(p<0.05). 2. Anterior area had more dimensional change than posterior area in same impression materials. In conclusion, thermal expansions of impression materials were significantly different from each other(p<0.05), and anterior area had more dimensional change than posterior area in same impression materials.