Purpose:The present study was to apply the Attitudes-Social influence-Efficacy (ASE) model in order to identify factors associated with the assertive behavior of non-smoking college students when they are exposed to secondhand smokes in Korea. Methods: Data were collected from non-smoking college students (N=1,656, 76.6% female) at two universities in Seoul and Gyeonggi-do. The main outcome measure was the assertive behavior. ASE factors such as attitudes toward being assertive, social influences, and self-efficacy as well as socio-demographic, health-related, and smoking-related factors were self-administrated. Results: The mean of the assertive behavior (range 1~5 points) was 2.23;37.6% was not at all assertive, while 4.3% was always assertive. Higher assertiveness was significantly correlated with a higher level of positive attitude, social influence and self-efficacy (p<.05 for all). Stepwise multiple regression analyses showed that the social influence was the strongest factor associated with the assertive behavior (${eta}=0.430$, p<.001, $R^2$=.246), followed by self-efficacy, motive to assertiveness, having any family member who had diseases, and male gender. These factors explained the assertive behavior by 39.7%. Conclusion: The ASE model may explain the assertive behavior of non-smoking college students under secondhand smoke exposure. Social influence and self-efficacy were significant factors associated with their assertive behavior.