Abstract: Objective　To study the effect of shortening the time of etching on the bonding strength of primary teeth and immature permanent teeth to provide some theoretical guidance for more proper control of acid etching time clinically. Methods　20 fresh and not crack primary molars, 20 not crack immature permanent teeth were selected and the enamel of tooth surface was grinded to form the dentin plane. Primary teeth were randomly divided into A, B, C, D group and immature permanent teeth were divided into E, F, G, H group. Group A and E should be etched for15 seconds by SB2, 7 seconds for group B and F. Group C and G should be etched by OV for 20 seconds, and 10 seconds for group D and H. Then layered resin was solidified and the crowns were restore. Finally, the teeth were cut piece into a cuboid shape sample with a cross-sectional area about 1.0 mm×1.0 mm and then micro tensile strength test was carried out. Results　Micro tensile strengths respectively were group A（14.95?3.79）MPa, group B（16.74?2.77）MPa, group C（20.80?3.97）MPa, group D (20.50?3.98)MPa, group E（20.95?2.68）MPa，group F（17.26?3.32）MPa，group G (25.70?2.95)MPa, group H (20.98?3.13)MPa. There was no significant difference between group A and group B ,group C and group D. Group F ＜ group E and group H ＜group G, the difference was statistically significant(P ＜0.05); Group C ＞ group A; group D ＞ group B; group G ＞group E; group H ＞ F group, the difference was statistically significant(P ＜0.05). Conclusions　Shortening the acid etching time, the bonding strength of the primary teeth did not change significantly, but the bonding strength of the immature permanent teeth significantly reduced; the bonding strength of applying OV is higher than that of the application of SB2 .

Key words: primary teeth, immature permanent teeth, Microtensile strength