We examine educational gradients in depression and identify underlying mechanisms of how education might affect depression. We use a nationally representative sample of community-residing adults aged 45 and older from the 2006 Korean Longitudinal Study of Aging, which collected information about depressive symptoms and education. Using tobit regression, we estimate the effect of education on depression and examine what can explain the education gradients by controlling for proxy variables of different pathways linking education to depression. We found cognitive ability, economic resources, social status, social network, and health behavior explain all of the education gradients. Education affects depression through different underlying mechanisms, and the single most important pathway is through developing cognitive ability. Through these pathways, educational attainment influences not only depression for an individual but also for one's spouse, particularly for women, and parents.