The purpose of this study was to clarify the effects of peroxide on teeth after the tooth-whitenerapplication. An artificial root canal was prepared using the crown of human incisors filled with phosphate buffer solution. Tooth-whitener containing 30% hydrogen peroxide or 35% carbamide peroxide was applied according to the instructions with or without acid-etching pretreatment. Luminol-chemiluminescence was used to measure the quantity of peroxide that penetrated into the buffer within the root canal. The penetrated peroxide quantity ranged from 7 μM to 100.5 μM. The penetration quantity for the hydrogen peroxide-applied group was about 1.6-fold greater than that for the carbamide-applied group. The penetration quantity for the acid-etching pretreatment group exceeded about 4-fold greater than that of the non-pretreatment group. The whitener containing hydrogen peroxide consists of liquid and powder. Qualitative analysis of powder component of the whitener with EPMA showed that it contained Fe and Cu ions. This finding suggests that whitener affects on the tooth by means of hydrogen peroxide itself or the oxygen radical that generated with Fe or Cu ion. Hence, these findings suggest that the penetration of peroxide from whitener applied to the tooth surface might cause some discomfort after whitening treatment.