Objective:To investigate the effect of variable etch times on the surface structure characterization of immature permanent and permanent teeth. Methods:We collected 90 premolars extracted for orthodontic need,in which immature permanent teeth (patients aged 12 to 17) were 45 and permanent teeth (patients aged 18 or higher) were 45. The differences of enamel surface microtopography between immature permanent and permanent teeth after variable etch times were observed under scanning electron microscope (SEM),and the changes of enamel surface element content,microhardness,shear bond strength,and index of ARI,etc. were analysed. Results:SEM showed that the interprismatic substance of immature permanent teeth has started to dissolve after 15 s. The width and height of prism both decreased after 30 s. With the increase of etch time,substance dissolved gradually and normal structure of enamel prism was destroyed by acid step by step. After 60 s, normal structure of the enamel surface almost disappeared. The interprismatic substance of permanent teeth dissolved obviously after 30 s. No significant difference of the enamel surface morphology was observed with the time increased. The value of surface microhardness of permanent teeth was significantly greater than that of immature permanent teeth. After variable etch times,the shear bond strength of immature permanent and permanent teeth was of no significant difference. The calcium and phosphorus content and Ca/P ratio of immature permanent teeth showed no statistical difference,however,after 60 s the phosphorus content of permanent teeth was much smaller than that of the 45 s group,Ca/P ratio of the 60 s group was less than that of the 30 s and 45 s groups. Conclusion:In order to reduce the etched damage to the enamel surface,etch time of immature permanent teeth in the orthodontic clinic can be appropriately reduced to 15 s,and permanent teeth can be extended to 30 s.