Purpose: This study was done to identify predictors differentiating the preparation stage, which is the stage that the smoker is ready to quit smoking, between adolescent smokers and adult smokers. Method: A survey was conducted with 376 adolescent smokers in 4 high schools and 451 adult smokers in community settings in South Korea from August 2003 to April 2005. To identify the predictors before and after preparation in stages of change of smoking, logistic regression was done. Results: The predictors for before preparation in stages of change of smoking were process of change for smoking abstinence for adolescent smokers and depression for adult smokers. The predictors for after preparation in stages of change of smoking were self-efficacy for smoking abstinence for adolescent smoker and self-efficacy for smoking abstinence and smoking temptation for adult smokers. Conclusion: For each group, adolescent smokers and adult smokers, specific smoking intervention methods need to be developed based on the different ways individuals make the decision to quit smoking within their contexts.