Abstract: Objective The aim of our study was to adopt histological observation of effects of bleaching agents in high concentrations on tooth pulp of rats Methods 60 rats were divided into three groups: bleaching with NaF group, bleaching group and control group. Rat teeth in two experimental groups were bleached for 5 minutes, 10 minutes and 15 minutes. Every 5 rats were killed after bleaching immediately, 3 days and 7days after bleaching respectively. Then histopathologic examination of pulp tissue was taken. Results Slight symptoms were observed in one third of all the teeth. No moderate or severe symptoms were observed in all the teeth after bleaching. Slight symptoms could be observed immediately after bleaching, and they generally disappearred 2 weeks later. There were no significant differences berween two experimental groups. Conclusion The findings from this study demonstrated that HP type tooth bleaching treatment in high concentrations might cause mild, localized pulp reactions. No significant changes could be observed when fluoride was used before bleaching treatment.