The article discusses the role of third-party strategies in resolving interpersonal conflicts, by which we mean the participants choosing to cooperate. In the experiment, three strategies of third-party behaviour were modelled – cooperative, neutral and aggressive. The study involved 156 subjects, 95 girls and 61 boys, all 10-11 grade high school students. 18 trained research assistants played the role of the third party. The study was conducted in two series corresponding to the two experimental conditions – in a group and in dyads due to the differences in the conflict parties’ behaviour. The essence of the experiment was to introduce a conflict situation by creating competition between the parties. This was done as part of a game through a simulation of struggle for a limited resource, which in our study was the highest grade and depended on the solution of biological crossword. Results of the study showed that the third-party strategy did have an impact on the process of resolving interpersonal conflict. It has been proved that there are differences in the effectiveness of third-party behaviour depending on the conditions of the conflict – in a group or in a dyad. The results showed that the aggressive strategy of third party encouraged participants to cooperate in a dyad within a social group. The results can be used to create guidelines for teachers, often acting as a third party in the resolution of conflicts, and for the adolescents themselves.