Bleaching of teeth is primarily indicated for cosmetic purposes. Numerous carbamide peroxide preparations have been introduced, including variations on the dentist-prescribed/home-applied techniques, as well as over-the-counter bleaching kits sold directly to consumers in stores. Objectives: this in vitro study was to compare, using digital photography, the efficacy of two tooth-whitening gel products: the dentist-prescribed type (Opalescence TM) and the over-the-counter type (Sparkle TM). The whitening efficacy was evaluated by measuring the value of L (black-white), *a (green-red) and *b (blue-yellow). Methods: seventy-one recently extracted premolars for orthodontic purposes were intentionally darkened by applying Ledermix paste into the pulp chambers for two weeks. Each tooth was sectioned longitudinally into two parts. The cutting surfaces were sealed with glass ionomer cement. One part of each tooth received Opalescence TM (66 half-teeth), another part received Sparkle TM (66 half-teeth) for two weeks of bleaching. The other 10 parts received no bleaching treatment and served as a control. Then, the specimens were stored in artificial saliva for observation of tooth color at 3 and 6 month intervals. Pictures were taken and the color changes of the tooth samples were compared between before/after 2 weeks, and at the periods of 3 and 6 months after bleaching. Results: they indicated that after 2 weeks of bleaching, both gels whitened teeth significantly (p<0.05). The ability to lighten the teeth of both gels was not significantly different (p>0.05). There was no difference in color change in the two groups at 3 and 6 month lapses. (p>0.05). Conclusions: these findings suggested that under the circumstances both the over-the-counter and the dentist-prescribed bleaching gels produce the same whitening efficacy and 6-month color stability. "Clinical significance:" The whitening efficacy is not different significantly for the OTC and the dentist-prescribed.