Abstract: Objective To investigate the prevalence of depression among only-child and non-only child middle school students in Yunnan province and to provide evidence for depression intervention. Methods Center for Epidemiologic Studies Depression Scale(CES-D),Social Support Rating Scale(SSRS),Middle School Students Coping Style Scale,and a selfcompiled questionnaire were used to survey 12 206 students selected with multi-stage stratified cluster random sampling from 17 middle schools in 6 city,county or district in Yunnan province. Results The prevalence rate of depression in the onlychild students was 21.5%,which was lower than that of the non-only-child students(24.6%).Gender,free after school time,self-reported family income,study stress,and other life events were important factors affecting the prevalence of depression. Conclusion The mentl health status of non-only child middle school students in Yunnon province in not good and considerations should be paid to the students to promote their health condition.