The purpose of this study was to evaluate the effect of heat pressing on the 3-point flexural strength of zirconia. To prepare sixty zirconia plate, Zirtooth™ block(?98×14T, HASS, Gangwondo, Korea) were cut into 24㎜ long, 4㎜ width, and 0.5㎜ thick after sintering at 1,450℃. Specimens were divided into 6 groups depending on the investement materials as follows; (a) UN group (Control), (b) PH group (Prime vest HS), (c) CP group (Calibra-press), (d) BV group (BC-Vest), (e) MH group (Microstar-HS), (f) F1 group (Formula 1). Wax patterns with a size of 24㎜ long, 4㎜ width, and 1.0㎜ thick were attached on the zirconia specimens. They were invested using five investment materials according to the procedure recommended by the manufacturer and then left at room temperature for 30 minutes. The invested mold was dried and maintained at 850℃ for 50 minutes. Then, Amber Lisi-POZ LT(HASS, Gangneung, Korea) was placed in a thermoformed electric furnace (Programat EP3000/G2, Ivoclar vivadent, Schaan, Liechtenstein) with the mold, heated at 915℃ for 15 minutes after rising the temperature with heating rate of 45℃/ min. 3-point flexural test of 3Y-TZP specimens were performed at crosshead speed of 0.5 ㎜/min. Surfaces of zirconia specimens were observed by a Field-emission scanning electron microscopy (FE-SEM) and analyzed by energy dispersive X-ray spectroscopy (EDS). 3-point flexural strength was the highest (1265.5㎫) in UN group and was the lowest (756.1㎫) in the F1 group. The largest amount of Si was detected and the most interfacial changes occurred in the F1 group. When the zirconia was buried with the investment material and then the heat press molding was performed, the structural change of surface was occurred by the chemical reaction between zirconia and investment material, which resulted in strength reduction of zirconia.