This paper concerns the structure of Korean business groups. We investigate the factors that affect a controlling shareholder's decision regarding the structure of his business group using financial and ownership data on conglomerate groups in Korea. We define new measures that represent the levels of vertical and circuitous structures of a group. We empirically confirm that controlling shareholders strategically choose the structure of their business groups to secure control over the groups and to seek private benefit of control. The risk diversification and propping incentive of controlling shareholders is also found to affect the decisions.