The purpose of this study was to measure thermal expansions of dental investments, Biovest(Casting Investment. Dentsply International INC, U.S.A.), Multi-Best (Use for all dental chrome-cobalt alloys, The Ransom & Randolph Co. U.S.A.), Kerr(Inlay Investment. Sybron Kerr, U.S.A.), O. K. (Inlay Investment. Shofu Dental MFG, Co. Japan), Whip-Mix (Cristobalite Inlay Investment. Whip-Mix Corporation. U.S.A.). Thermal expansion of specimens(5mm in diameter and 50mm in length) was measured by a dilatometer at the temperature range from $20^{circ}C$ to $700^{circ}C$ by comparing expansion between standardized quartz and experimental specimens with heating rate about $300^{circ}C$/hr. The following results were obtained. 1. The coefficient of thermal expansion of Biovest was $15{ imes}10^{-6}/^{circ}C$ in the water powder ratio 18/100 and $14{ imes}10^{-6}/^{circ}C$ in the water powder ratio 28/100. Those of Multi-Best were $9{ imes}10^{-6}/^{circ}C$ in the water powder ratio 14/100 and $7{ imes}10^{-6}/^{circ}C$ in the water powder ratio 24/100. 2. The coefficient of thermal expansion of Kerr were $17{ imes}10^{-6}/^{circ}C$ in the water powder ratio 38/100 and $14{ imes}10^{-6}/^{circ}c$ in the water powder ratio 48/100. Those of O. K. were $9{ imes}10^{-6}/^{circ}C$ in the water powder ratio 33/100 and $7{ imes}10^{-6}/^{circ}C$ in the water powder ratio 43/100 3. The coefficient of thermal expansion of Whip-Mix were $14{ imes}10^{-6}/^{circ}C$ in the water powder ritio 40/100 and $12{ imes}10^{-6}/^{circ}c$ Fein the water powder ratio 50/100. Those of Hi-Heat were $11{ imes}10^{-6}/^{circ}c$ in the water powder ratio 28/100 and $10{ imes}10^{-6}/^{circ}c$ in the water powder ratio 38/100.