Aims: To determine if the composite resin restoration need to be replaced after power bleaching at different times.Materials and methods: Three composite resins (a flowable, a conventional and ceramic composite) were used for this study. Transparent discs with 2mm thickness with a hole of 5mm diameter were used for the production of composite samples, six samples were made for each type of composite with a total of 18 samples were used. The bleaching procedure was followed on the top surface of each sample. For each sample a newly fresh material were mixed (lase peroxide sensy gel) and applied over the prepared sample. Bleaching took place for 15, 30 and 45 minutes according to the manufacturer instructions. For the microhardness measurements, a Vickers microhardness tester (Wolpert, Germany) was used, with a 200g load. Five microhardness measurements were obtained on the top surface of each sample on the following time periods: before bleaching, after 15 min., after 30 min. and after 45 min. of bleaching. Data were analyzed using T-test, a value of p≤0.05 was considered significant followed by the analysis of variance (ANOVA). Results: The results showed a significant increase in the microhardness of all the three types of composite resins after bleaching with a different time when compared to the control.Conclusion The bleaching agent that was tested has a significant increase of the microhardness of the composite resins and there is no sufficient reason to indicate the replacement of restorations.