Introduction: Mouthwashes which prevent and control caries and periodontal diseases are commonly used even without professional prescription. Long-term use of mouthwashes may cause softening of restorative composites and lowering the longevity of restoration. The aim of this study was evaluation of surface hardness value of a microhybride composite (Filtek Z250) after 12 hours immersion in three kinds of alcohol-free mouthwashes. Materials and Methods: 72 Cylinderical specimens of a microhybride composite 5mm wide were prepared, using drinking straw as a mold. Specimens were light-cured continuously for 40 seconds on each side with Elipar(3M,ESPE) curing light. The specimens were immersed in 50ml of distilled water for12 hours. After that, all of them were finished with silicone carbide papers under constant water as coolant. The specimens were divided into 4 groups, each with 18 samples, the first group immersed in Colgate plax , the second in Crest (pro-health for me) , group3 in OraCare and group4 in water as the control group for 12 hours, which is equivalent to 1 year of daily mouthwash use at 2 minutes per day. Hardness measurement was taken by Vickers hardness tester with 1 kilogram load and 10 seconds loading time. Result: Statistical analysis according to t-test and One-Way ANOVA test showed that there was no significant difference in surface hardness value of composite after 12 hours immersion between groups of mouthwashes and water(P value=0.353) Conclusion: Based on the present study, alcohol-free mouthwashes didn't affect the surface hardness of Filtek(Z250) composite.