Abstract： Objective To observe the microstructure changes of rat alveolar bone around tooth root under orthodontic force loading. Methods Ten 10-week-old rats were included in the study. Upper first molars were moved mesially with 0.196 N of force. The alveolar bone around the root of upper first molar was scanned by a micro-computed tomography scan system(SkyScan-1076) in different period after the initiation of orthodontic force loading( on the 3rd, 7th, 14th day after force loading ) and analyzed by a speciallydesigned software to measure the microstructure parameters of alveolar bone ( bone volume fraction, bone surface to volume ratio, structure model index, trabecular thickness, trabecular separation, trabecular number). Results From the 7th day, bone volume fraction[(41±14)%], structure model index( 1.51 ±0. 52) and trabecular separation [(90 ± 30 ) μm] changed significantly in the compressive area compared with those[(64±15)%, (0.51 ±0.85), (56 ±-10) μm] on the 3rd day. From the 14th day, bone volume fraction[(78 ± 14)%], structure model index(0. 28 ± 0. 20) and trabecular separation[(29 ±13) μm] changed significantly in the tension area compared with those[(67 ± 14)%, (0.40 ±0.41 ),(48 ± 15) μm] on the 7th day. No difference on trabecular number was found between the compressive area and tension area(P ＞0. 05). Conclusions The significant resorption of the alveolar bone was observed on the 7th days in the compressive area and the deposition of the alveolar bone was observed on the 14th day in the compressive area after orthodontic force loading.