The present study examined the hypothesis that industrialization has negative effects on mental health. Based on the hypothesis, it was predicted that the mental health of the dwellers in a large city is poorer than that of the dwellers in a small city. The students in a Senior High School for Girls and in a Senior High School for Boys in Taipei city were used as subjects representing respectively for the large city dwellers of both sexes, and the students of a girls' and a boys' senior high schools in Yi-lan city were used to represent the small city dwellers of both sexes. The large and small city dwellers were comparedon the new form Ko Mental Health Questionnaire scores. It was foundthat: (1) the prediction was supported by the findings with the female as subject but not by those with the male as subject; (2) the large city female dwellers were easier to feel depressed and hostile; (3) the large city dwellers are less strict about time, money, sex, and aggression, and less concerned with the attitude of others toward them. Further studiesusing subjects of different ages and schools are recommended.