Against the background of remote work and labor-saving being promoted globally, the use of avatars is becoming widespread in our daily lives. Concurrently, the environments in which avatars are used are also diversifying, with environments appearing wherein communication is possible between humans and avatars as well as between avatars themselves. In this social situation, the effects of use of avatars on communication must be investigated. However, research to compare the effects of non-verbal information in avatar–avatar and human–avatar environments is inadequate. In this study, we created an avatar of which every facial feature can be moved independently and then measured the effects of facial expressions on communication in avatar–avatar and human–avatar environments. The results of a communication experiment based on negotiating in the context of the Prisoner's Dilemma game showed that mimicking facial expressions resulted in negotiations having a more cooperative outcome. Furthermore, the results suggest that this tendency is stronger in the avatar–avatar environment.