The purpose of this study was to investigate the influence of thermal cycling on the adhesion of the resin on the young permanent ground enamel which was etched with different etching times. Labial surfaces of 50 extracted and frozen bovine mandibular young permanent incisors were used. The etchant used in this study was 40% phospholic acid gel and the etching times were 0, 10, 20, 30 and 60 seconds. All of the specimens were washed with an air water spray after etching. The bonding agent and composite resin used in this study were Photo Bond and Photo Clearfil A (Kuraray Co.). After thermal cycling tests of 10,000 temperature cycles between 60 degrees C and 4 degrees C water baths, shear bond strengths on the ground enamel were measured. After the shear bond strength tests, all the test surfaces of the enamel and resin specimens were observed using the SEM. The results of this study were compared with the previous results which were not thermal-cycled. The following conclusions were obtained. 1) The etching time which showed the highest bond strength was 30 seconds (45.21 +/- 8.49 Mpa). 2) When the enamel was etched with all of the etching times, the bond strength were significantly higher than that of the enamel without etching. 3) In the etched groups, the bond strength with 20 seconds of etching time was significantly lower than those with 10, 30 and 60 seconds of etching times. 4) In the 10, 30 and 60 seconds etching times, the bond strengths with thermal-cycled groups were significantly higher than those without thermal-cycled groups. 5) On the enamel specimens after the shear bond strength tests, the frequency of the clear prism structures observed on the enamel was higher in the groups which showed higher bond strengths. However, the differences were not significant.