Cooperation is an important characteristic in some organisms, including humans, although such a behavior can be difficult to evolve and maintain. Typically, when all members of a population behave cooperatively, a defecting individual can enjoy a greater advantage by avoiding cooperative efforts. Such individuals are so-called “free rider” or “cheaters.” We are theoretically investigating the evolutionary process of cooperative interaction using mathematical modeling and computer simulation. We are currently studying three aspects of the evolution of cooperation: (1) relationships between structures of cooperation and variation of cooperation levels in a population, (2) the effects of negotiation between players on the evolution of cooperation, and (3) the joint evolution of resource exploitation and cooperation in twodimensional space. In the latter research project, we revealed that, in the spatial structure, the evolution of cooperation can be facilitated by the evolution of resource exploitation that is accompanied by a competition among individuals (Fig.1). This indicates a paradoxical feature in the evolution of social interaction, i.e. a positive interaction (cooperation) can be evolutionarily promoted by the simultaneous development of a negative interaction (exploitation) in a spatially structured population.