Abstract: Objective To explore the value of Modified Overt Aggression Scales （MOAS） in assessing risk behavior of patients with mental illness. Methods Four hundred and ninety patients, who were from three mental health institutions （Refined Control Ward in Ankang Hospital, Mental Health Center of West China Hospital, Relief Ward in De Kang Hospital） belong to public security, health and civil system respectively, were evaluated with MOAS. Results Three hundred and thirteen patients did not have dangerous behavior, but 177 patients did （99 aggression, 78 violence）. The descending order of risk behavior in patients with mental illness were Refined Control Ward in Ankang Hospital, Mental Health Center of West China Hospital, Relief Ward in De Kang Hospital. Physical and total aggression scores were higher in Refined Control Ward in Ankang Hospital than that in other two hospitals（P<0.05）. Autoaggression score in Mental Health Center of West China Hospital was higher than that in other two hospitals（P<0.05）. Physical and total aggression scores in male were higher than that in female（P<0.05）. Autoaggression score in female was higher than that in male（P<0.05）. Score of every single item and total score were higher in violent and aggressive group than that in non dangerous behavior group（P<0.05）. Physical and total aggression scores in violent group were higher than that in aggressive group（P＜0.05）. Conclusion The proportion, type and degree of risk behaviors in three mental health institutions are different and the type of risk behavior has gender difference.

Key words: forensic psychiatry, risk behavior, risk assessment, Modified Overt Aggression Scales