Background & Aim: Considering the widespread use of composite restorations and the provision of nanocomposites on one side and the common use of mouthwashes, there are concerns about the microhardness of composites and their known complications following mouthwashes. So, the present study was aimed to compare the effect of two mouthwash with Alcohol and without Alcohol on microhardness changes of two types of composite resins. Material and methods: Experimental design was performed. In total, 32 samples of composite specimens of 3 mm in diameter and 3 mm thick were prepared from nano composites Z350 (3M ESPE, USA) and Z250 (3M ESPE, USA). The primary hardness was measured by the Matsuzawa japan machine (Vickers hardness tester), and then the specimens were randomly divided into two subgroups. (n = 8) were then inserted into 20 ml of Alcohol-free and non-Alcoholic listerine mouthwash for 24 hours, then removed the specimens and the secondary microhardness was measured by the apparatus. Microhardness changes in each group were performed by paired Ttest test. Results: The highest microhardness changes in the Z250 micro-hybrid composite in the mouth of the listerin containing Alcohol was 11. 4 ± 3. 4 and the lowest was related to the Z350 nano-fiber composite in the listerine-free zinc mucous membrane at 0. 4 ± 4. 2. Mouthwashes containing Alcohol significantly reduced microhardness in both composites (P <0. 001). Conclusions: It seems that mouthwash containing Alcohol significantly reduces the microhardness of composites. In terms of microstructure, there is no significant difference between the Z350 nanofill composite compared to the Z250 micro hybrid composite when it is placed in the mouthpieces but Z350 nanofill composite showed better functions.