Background: Varenicline is an effective smoking cessation aid. However, smokers prescribed with varenicline do not always receive varenicline for 12 weeks, as recommended. This study analyzed the subjects who received varenicline and investigated the effect of varenicline treatment duration on the success rate of 6-month smoking cessation. Methods: This study retrospectively analyzed 78 subjects, who received varenicline, out of the 105 smokers that had visited the smoking cessation clinic after medical examination from September 2007 to December 2009. Results: The subjects were all males. Twenty-two subjects (28.2%) had varenicline treatment for 12 weeks or longer; 18 subjects (23.1%) for 8~12 weeks; 22 subjects (28.2%) for 4~8 weeks; and 16 subjects (20.5%) for less than 4 weeks. The total success rate of the 6-month smoking cessation was 47.4%. The success rate of the 6-month smoking cessation was 63.6% in the group that received varenicline for 12 weeks or longer, which was higher than 41.1% of the group that early terminated the varenicline treatment (p=0.074). The period of varenicline treatment was extended for one more week, the odds ratio of the 6-month smoking cessation success increased to 1.172-folds (p=0.004; 95% confidence interval, 1.052~1.305). Adverse events occurred in 30.8% of the subjects who received varenicline, but no serious adverse events were found. Conclusion: If varenicline treatment period is extended, the odds ratio of the success rate for the 6-month smoking cessation increases. Therefore, an effort to improve drug compliance for varenicline in clinical practices could be helpful for the long-term success of smoking cessation.