This article analyzes Japan's dual role in the Kyoto Protocol negotiations. It argues that the successful conclusion to the 1997 Kyoto Protocol can be partly attributed to Japan's dual role as negotiator and mediator. As the host of the Kyoto Conference, Japan undertook the mediator's role prior to and during the conference negotiation process. At the same time, as a participant to the conference, Japan took a negotiator's role to meet its own interests in the negotiations. Despite its initial position as a self-interested negotiator like any other country, Japan ended up in a different position than expected because of its broader role as mediator. By reviewing Japanese efforts to bring the protocol negotiations to a successful conclusion, the article examines the dynamics of the two dimensions of mediation and negotiation in international treaty-making.