It is a recent trend to emphasize community services for the rehabilitation of the mentally ill. However, it is also necessary to provide social work services for those living in the institutions, who is the most underprivileged in our society. To deal with interpersonal skill deficits which is one of the main characteristics of the mentally ill, social skills programs are being utilized, usually in the communities, but not in the institutions. Therefore, the objective of this study is to develop and evaluate institution-relevant program for the improvement of assertiveness, which is the basic ability among the interpersonal skills, and self-esteem, which is the emotional basis of assertiveness. Because there has been little literature on such a program, a preliminary program had been implemented in order to include the peculiarities of the life in the institutions on the main program. The evaluation of the program effect was done by pretest-post test control group design, and its results are as follows. Firstly, in regard to program effectiveness, the pre and post test showed that self-esteem improved in a statistically meaningful way, but that assertiveness did not so. However, the staff verified the improvement of the group members' assertiveness to some degree in their actual lives. Secondly, in regard to the group process, the group interaction improved in a statistically meaningful way between 2nd and 5th session, and the improvement maintained till the program termination. Also the group members evaluated the program as helpful in their everyday lives. Therefore it can be said that the result of this program is encouraging. In light of this, several suggestions were made: utilizing more dynamic techniques in the program; developing the advanced level of the program; making an effort to change environmental structures as well as individuals; applying this kind of program to another living facilities such as those for the homeless or handicapped.