Although motivational interviewing (MI) has been shown to be effective in changing health behaviors, its effects on smoking cessation have been mixed. The purpose of the present study is to assess factors of motivation and self-efficacy as they mediate the relationship between MI and smoking cessation. This is a secondary analysis of an MI based smoking cessation randomized trial. MI counseling was associated with change in smoking behaviors during a 12 months intervention but was not related to autonomous motivation, controlled motivation, or self-efficacy at baseline and 6 months, the hypothesized mediators. This study confirmed the pathway to quit smoking through increase in self-efficacy.