Aim: The aim of this study was to evaluate the effect of thermocycling on microhardness of three composites cured at different durations. Materials and methods: One hundred and twenty disks (N = 120) were made with a dimension of 5 mm diameter and 2 mm height using a metal mold from three composites (Filtek Z350 XT, Tetric N Ceram, Z100 Universal Restorative). Forty disks each were made with each composite, which were again divided into two subgroups according to the duration of curing, 20 and 40 seconds. Microhardness test was done before and after thermocycling using Vickers microhardness test. Thermocycling was done at –5 and at 55°C for 250 cycles. Values were analyzed using analysis of variance (ANOVA) test, Tukey's simultaneous comparison. Results: Among the groups cured for 40 seconds, in the control group, Z100 restorative showed higher microhardness. After thermocycling, even though there was a significant reduction in microhardness in all three composites, Tetric N Ceram showed minimal change in values compared with the other two composites. Conclusion: It was concluded that after thermocycling, the composite resins are susceptible to significant reduction in microhardness. Clinical significance: Patient's oral conditions, such as the cyclic thermal gradient and humidity can have negative effects on the mechanical properties of restorative materials. This is one of the factors contributing to a future need for replacement of dental restorative materials.