Objective: To evaluate the effect of thermocycling on microleakage of cervical cavities restored with hybrid and flowable resin composites polymerized with 3 different light-emitting diode (LED) curing techniques. Method and Materials: A total of 120 wedge-shaped cervical cavities were prepared on the buccal surfaces of human molars, restored with hybrid resin composite (Z250, 3M Espe) or with flowable resin composite (Filtek Flow, 3M Espe), and cured with either an LED standard light, an LED high-intensity light, or an LED exponential mode, which is a kind of soft-start curing method. The teeth in each group were divided randomly into 2 subgroups: (1) thermocycling for 1,000 cycles at 5°C and 55°C with a dwell time of 60 seconds, and (2) no thermocycling treatment. All teeth were immersed in 0.5% basic fuchsin dye solution for 24 hours. The teeth then were longitudinally sectioned and scored on a 0 to 3 scale at the enamel and dentin margins. Results: Nonparametric statistical analysis of the results showed a significant difference in microleakage between the nonthermocycled and thermocycled specimens, except for specimens cured with soft-start polymerization. Conclusion: The soft-start polymerization technique with LED light was not affected by thermocycling regimens. Keywords: curing methods, LED, microleakage, resin composite, thermocycling