Eating disorders (ED) including anorexia nervosa (AN) and bulimia nervosa (BN) are stress-related disorders that strongly disturb the patients' life. Their low assertive behavior is thought to be connected with low self-esteem and pathogenesis of ED. However, none of the studies has conducted assertion training with cognitive-behavioral therapy (CBT) for ED. We hypothesized that the acquirement of assertive behavior would influence their self-esteem and psychological state, which would then lead to reduction of abnormal eating patterns. Seventeen patients with eating disorders were allocated in this study. Ten patients were assigned to group-CBT program and seven patients to the control group. Psychometric tests, behavioral assessment, body mass index (BMI) were investigated before and after the treatment. ANOVA indicated a significant group ×period interaction in state anxiety (p <0.05), suggesting remarkable reduction of anxiety in patients treated with group-CBT. Anxiety management is an important factor for acquiring the assertive behavior. Thus, it is necessary to increase the number of cases and to repeat the further studies. Group-CBT may be a new effective treatment for ED.