Family dissolution and economic difficulties of the grandchild's parents are the most prevailing reason for an increase of grandparent-headed families in rural Korea. However, there is a lack of empirical research looking at the experiences of custodial grandmothers, social support, and their psychological well-being. In this regard, this study examined the effect of social support (formal & informal) on the psychological well-being of custodial grandmothers in rural Korea. For this purpose, quantitative and qualitative research methods were conducted. Quantitative data were gathered, using structured questionnaire, from 311 grandmothers who were raising their grandchildren in rural Korea as primary care-givers. Statistical methods used for data analysis were descriptive statistics and hierarchical regression analysis with SPSS WIN 19.0 programs. Qualitative data were collected by in-depth interviews to 10 custodial grandmothers in rural area. The results of this study are as follows. First, almost half of the parents of the grandchild didn't visit and didn't call anymore. Some relatives -aunt, uncle, etc.- support the grandparent-headed family economically and emotionally. 3/4 of the grandparent-headed families were supported by the government and community. This was not small numbers, but the amount of economic support was low. Second, grandmothers in the study showed quite a high level of depression while their happiness level was relatively low. There were differences in the factors influencing the level of happiness/depression of these rural grandmothers. Overall, the study's results showed the importance of formal & informal support for psychological well-being of the custodial grandmothers in rural Korea. Policy and theoretical implications are discussed.