Objective: One of the major concerns with composite inlays is the long term gap formation. The purpose of this study was to show the effect of thermocycling on the gap formation between composite inlays (Z 250, 3M ESPE) and new luting agents (RelyX Unicem (RLX), 3M ESPE; Calibra (CAL), Dentsply/Caulk; Panavia F (PNF), Luraray, Fuji Plus (FJP), GC). Methods: A cylindrical cavity was prepared in the dentin cavity, having a diameter of 3.5 mm with a depth of approximately 1.5 mm. The composite inlays were inserted using four luting agents after ptretreatment according to the manufacturer's instructions. A restored tooth was polished and examined after one-day storage (Base-line). Then the presence or absence of gaps was measured at 14 points (each 0.5 mm apart) along the cavity restoration interface (n=10, total points measured=140, X 1,000, Oper Dent 2002; 27:59-65). The same samples were re-measured after 10,000, 20,000 and 40,000 thermocycles (TC) (5 or 55 C). Statistical analyses were done in the Tukey test (Non parametric, p=0.05). Results: Results summarized data for gap formation in the restorative cavity: Sum of Gap Formation (n=10, Total measuring points=140) RLX CAL PNF FJP Base-line 8 A 12 B 19 C 12 D After 10,000 TC 10 A 16 B 21 C 11 D After 20,000 TC 16 A 21 B 30 C 28 D E After 40,000 TC 18 A 32 B 43 C 44 E The values designed with same letters are not significantly different. Conclusions: 40,000 thermocycles did not significantly affect gap formation of the three resin luting agents with composite inlays. However, it did significantly affect the tested resin-modified glass-ionomer luting agents in the negative way.