It is important for psychiatric nurses to understand client`s behavior thus developing effective relationships between clients and nurses. This study was conducted to designing an assessment tool, which is helpful to nurses in the psychiatric mental-health setting. This tool, based on Roy` adaptation model, is adopted the various nursing setting and validates the applicability. The research was doing from Dec. 1995 to Jan. 1997. During the first stage, this tool was designed through literature review and nursing history form, using one national psychiatric hospital, five university affiliated hospital, and a neuropsychiatric ward. Secondly, the tool was distributed to psychiatric nurses, who were asked to fill out the form and write the strength and weakness after having been interviewed. The last stage, incorporated a case study using this tool. The evaluation of this tool helped the researcher gather much of data required to make a specific nursing diagnosis. Finally, a psychiatric nursing assessment tool was designed to identify the four adaptation levels. The positive responses included the ability of the nurses to ask specific questions and guide the direction of interview, systematically by four adaptive modes. A number of nurses stated in the evaluation meeting that this tool helped them to get to know the patients more fully and to broaden their perspective in nursing assessment. The negative responses were : the tool was too long to provide an effective interview and was not clear regarding the boundaries of the three modes ; self-care, interdependence, and role function modes. Consequently, the redundant items was partially excluded. The check list form was included in physiological mode for time and anti-psychotic drug` side effects which cause severe trouble in therapeutic setting was added in physiological mode-neurological. The self-concept mode items including mental status examination, have a high relevance with mental health assessment. Finally, further study is needed to increase the effectiveness and adequacy of this tool.