Abstract: Objective The purpose of this research is to discuss whether the erupting of the third molar makes the first molar′s mesial movement easier during the beginning of the orthodontic treatment. Methods 18patients aged from 18 to 24 were selected. All the patients were treated with 4 premolars extracted, except for unilateral removal of a third molar randomly, other conditions for both side remained identical. After 3 months′treatment, the movement of the mesial-buccal cusp of the maxillary first molar was measured in both sides. Results 1）There was no significant change in mesial movement of the maxillary first molar in the extracted side during the 3 months′ treatment（P >0.05）. 2）There was significant change in mesial movement of the maxillary first molar（1 mm） in the non-extracted side during the 3 months′ treatment（P<0.01）. 3）The distance of the mesial movement of the first molar in the non-extracted side was larger than the extracted side，and the difference was
significant（P<0.01）. Conclusion In the beginning of the orthodontic treatment, the erupting maxillary third molar will push the first molar to move mesially while there was not enough erupting space for the maxillary third molar.

Key words: third molar')">third molar, first molar, mesial movement, anchorage loss