The aim of the present study was to evaluate the bleaching hypersensitivity of two bleaching systems (Flash and Zoom) during and one week after the bleaching session. Methods: A total of 40 patients age (20-40) were selected from the outpatient clinic of the Faculty of Dentistry, M.S.A University. The patients were divided into two groups, 20 patients for each bleaching system, either Flash or Zoom. Each bleaching system was applied according to the manufacturer's instructions. During each bleaching session each patient was given a chart to record the degree of teeth sensitivity during each session. In addition, after finishing the bleaching sessions each patient was given another chart divided into seven days and each day the post bleaching hypersensitivity was also recorded. The hypersensitivity was given a score from 0 (no post bleaching hypersensitivity) to 10 (very high). Results: Statistical analysis of this clinical trial revealed that there was no statistically significant difference between the two bleaching systems (Flash and Zoom). In both bleaching systems the post bleaching hypersensitivity was felt only for the first 24 hours after bleaching and scored a maximum scale of two.