One of the main concerns of policymakers regarding emissions trading markets is that some firms may well enjoy market power owing to their share of the emissions. This study shows that including indirect emissions within the coverage of an emissions trading scheme can help to reduce market power and thereby enhance social efficiency. In this study, the market concentration measured by the Herfindahl-Hirschman Index significantly drops after including indirect emissions in the South Korean emissions trading market. In addition, other market concentration measures are also considered to verify that the conclusion does not depend on the choice of concentration measures.