The phenomenon of familial clustering in depression is well established, yet the mechanisms by which depression is transmitted within families remain poorly understood. In the current study, we investigate the familial genetic and environmental transmission of depression by incorporating data from both adolescent twins and their parents. A total of 987 twin families were recruited from the Beijing Twin Study. Depression assessments were conducted for both adolescents and their parents. Twins' depression was assessed through reports from both the twins themselves and their parents, while parental depression was assessed by parental self-report. We employed a nuclear twin family model to examine genetic and environmental influences on adolescent depression. Our results, based on both self- and parent-report, demonstrate significant additive and dominant genetic influences on depression. We also found mild yet significant sibling environmental influences, while familial environmental influences were absent. Notably, parent-reported depression showed higher heritability but lower unique environmental influences compared with self-reported depression. These results highlight the important role of genetic transmission and sibling environmental transmission in explaining depression. Our study delineates the underlying mechanism of familial transmission in depression and can inform early treatments to halt transmission during adolescence.