In this research is shown part of results concerning evaluation study of group assertiveness training. Treatment, which has been organized as structured program of assertiveness training, has been applied on 18 groups of subjects during 9 group sessions. This treatment has been adjusted to non-psychiatric clients from our country. Groups were lead by two therapists. During 20 months of our research 215 voluntary participants of full age have joined training groups. Final sample consisted of 158 subjects who had finished treatment and who had fulfilled criteria for taking repeated measurement. Subjects had taken battery of questionnaires before the treatment and eight weeks after they had completed treatment. Subsample, which included 35 subjects from one therapist’s groups, took retest after twice longer time period. Results of conducted research have confirmed the hypothesis concerning the potentiality of assertiveness training to produce expected and significant improvements of treated subjects. Therapy effect is found on symptomatic level through reduction of symptoms of non-assertive behaviors, which were target of the treatment. Contrary to expectation interactive effect of treatment and therapist to reduction of symptoms of non-assertive behavior has been found, which implies need for further investigation of variables that are related to therapists. Through the medium of treatment therapeutic changes in structural level, in domain of general assertiveness, social anxiety and treated subjects' self-concept are produced. The effect of therapy we might consider stabled and maintained after 16 weeks of finishing the treatment.