Objective:To explore the effects of the microhardness of surfaces of composite resins by using three different light curing units at different temperatures.Methods:A total of 45 disc-shaped specimens were fabricated from Z250 composite resin(3M/ESPE,St.Paul,MN,USA) and were divided into nine groups by each of five samples.All samples in three different temperatures(frozen to 5 ℃,at room temperature 25 ℃,pre-heated to 37 ℃) was cured by a continuous halogen light(QTH),a softstart halogen light and a light emitting diode(LED).The microhardness of the top and bottom surfaces of the specimens was determined by using a Buehler Micromet II digital microhardness tester(Buehler,Dusseldorf,Germany).Data obtained was analyzed by using two-way analysis of variance(ANOVA).Results:As the temperature of composite resin increased,the top and bottom microhardness of the specimens also increased regardless of the type of polymerizing light used.The LED light produced a significant better hardness on top and bottom surfaces of composite resin specimens polymerized at three different temperatures.Effectiveness of cure at top and bottom surfaces of composite specimens was significantly reduced by using soft-start curing.Conclusion: The use of pre-warmed composite resins might help to improve polymerization of composite resin.LED light polymerization produces better results than halogen light.