Objectives: The purpose of this study was to evaluate the effect of different thermocycling stress on the marginal sealing and cavity wall adaptation of flowable composite restorations. Methods: Cylindrical cavities, 1mm deep and 3mm in diameter (C-factor=2.3) were prepared on flat bovine dentin surfaces. The teeth were restored with Clearfil SE Bond (Kuraray Medical) adhesive system followed by Photo Clearfil Bright (PB; Kuraray Medical) or Filtek Flow (FF; 3M ESPE) composites. The resins were cured with 600 mW/cm2 40 s using a light-curing unit (GC). The specimens were subjected into 2 different thermocycling protocols, Group 1: 500-cycles or Group 2: 5000-cycles between 5oC and 55oC with a 30s dwell time. After thermocycling, dye penetration tests around the cavity margin and cavity wall on cut surface were carried out. Dye penetration length was calculated as a percentage of the total cavity wall length. Dye penetration scores (n=8) were analyzed using the Mann-Whitney U test. Results: Dye penetration scores are showed in the table. Marginal dye penetration scores of 5000-cycles were significantly >500-cycles group in both PB and FF. Wall dye penetration scores of 500-cycles FF were Number of thermocycling Marginal leakage (%) Cavity-wall gap formation (%) PB FF PB FF 500 1.9 (3.7)a 0b 15.2 (8.2)c 4.4 (7.5)c, d 5000 12.6 (9.7)a 11.7 (6.1)b 15.5 (4.0) 16.5 (6.0)d Same superscript letters are significantly different. Conclusions: The flowable composite significantly improved adaptation of resin composite to the cavity wall under 500-cycles thermal stress condition. However, 5000-cycles thermal stress significantly decreased the cavity wall adaptation of flowable composite restorations. Supported by Grant #12307043 and #16591907 from Japan Min. of Educ. and COE Program, FRMDRTB at TMDU.