The essence of “subject of activity” concept is analyzed within the framework of methodological elaborations of subject principle developed by S.L. Rubistein and his followers. The peculiarities of the named concept is considered in conditions of its functioning in applied psychology related to person’s professional activity and development. It is shown that definition of subject of activity in use do not grow out of “specialization” postulate and “man and profession” system. But eliciting new property of the subject who achieved high professional level requires consideration of “man and world” system as well as “ontological subject” concept. This context is emphasized to find out the specter of different relations with environment such as active, cognitive, ethic and esthetic relations which highly qualified specialist actualizes in his professional activity.