The purpose of this study was to find out the social representation of mental illness in undergraduate students, patients and patient's family and compare the social representation of mental illness of these sample in 1976 with it in 1995. Participants were 418 normal person(undergraduate students and patient's family), 231 patients in 1976 and 342 normal persons, 159 patients in 1995. These participants completed the questionnaire consisted of 84 items about mental illness. The same questionnaire were performed both in 1975 and in 1995. the results of this study were as follows: 1) mental illness etiology: There were not changes in social representation about mental illness during the period of 20 years. Both participants in 1976 and participants in 1995 reported advanced civilization, bad social environment, weak willpower, parents' indifference toward the family as major onset reasons of mental illness. 2) mental illness symptom and diagnosis: Comparing with attitude in 1976, attitude toward psychiatric patient's thought, affect, and behavior was changed positively in 1995. 3) mental illness treatment and prognosis: Participants had positive attitude toward treatment possibility and prognosis. But they had social representations that mental illness was treated in the long time and gad difficulty in treating. 4) attitude toward mental hospital and patient: In the last 20 years, social representation of psychiatric patients was changed into sympathetic attitude. Also prejudice of mental hospital was mitigated. In the last 20 years, normal person's social representation of mental illness was changed largely. In perspective of etiology, symptom, diagnosis, treatment and hospital of mental illness, social representation was changed into positive attitude.