Abstract: Objective To evaluate the effects of the oriental preadjusted appliance-KOSAKA applied in treatment of mono-maxillary Angle class Ⅱ division 1 malocclusion with single maxillary extraction and discuss the mechanism and application. Methods 28 cases aged 17~26 years old with Angle class Ⅱ division 1 malocclusion were chosen and treated with the oriental preadjusted appliance-KOSAKA and with single maxillary extraction. Cephalometric analysis was performed before and after the treatment. Results In the sagittal direction, U1-NA and U1-SN were reduced 16.93° and 7.06°, U1-NA(mm) was reduced 3.83mm, U1-L1 was increased 12.94°, L1-NB and L1-MP were increased 2.78° and 4.62°, L1-NB(mm) were increased 1.50mm, and there were significant differences in these changes（P<0.05）, but the changes of SNA, SNB, ANB were no significant（P>0.05）. In the vertical direction, the changes of SN-MP and Y axis were no significant（P>0.05）.Conclusion The oriental preadjusted appliance-KOSAKA is effective for treatment of Angle class Ⅱ division 1 malocclusion with single maxillary extraction.