Professional's self-determination is intentionally originated and organized during organizational-activity game (OAG) imitating problem situations in different spheres of activity. Methodological principles of psychotechnic study using OAG based on L. S. Vygotsky's cultural-historical theory are discussed and its results are presented. Professional's self-determination is described as a phase sequence: each phase is characterized by specific processes occuring on self-determining pole as well as organizational means predetermining these processes proceeding.