Background: Smoking is considered the first preventable cause of mortality worldwide. There are many predictive factors on smoking cessation including social, family and individual issues. High nicotine dependency is one of the factors that make the cessation harder for the smoker. There are several methods for estimating the nicotine dependency rate in practice among which Fagerstrom test is one of the most suitable and non-invasive ones. Materials and Methods: This was a cross-sectional study. Data were collected from all smokers who had participated in the group therapy smoking cessation courses in Tehran smoking cessation clinic. The questionnaires were designed based on these data and nicotine dependency rate was evaluated via Fagerstrom test (FT). The smokers declared that they had quit smoking since the third session of the cessation course; this claim was confirmed by testing the expiratory carbon-monoxide rate. Finally, data were analyzed by using t-test and Chi-square test via SPSS software version 12. Results: There were 986 cases in this study including 786 (79.7%) males. Evaluation of the nicotine dependency rate showed that more than half of the smokers (n= 544, 55.2%) had high dependency, 330 (33.5%) had moderate dependency and 112 (11.3%) had low dependency. At the end of the course, 642 subjects quit smoking successfully. The maximum rate of success in cessation was among the smokers with low nicotine dependency (79 cases, 70.5%) and the minimum rate of success was among the smokers with high nicotine dependency (323 cases, 59.4%) (P=0.00). Conclusion: According to the results of this study smokers with high nicotine dependency had a low chance to quit successfully. Therefore, it is recommended to follow them up for a longer period of time and use combination therapy for them. (Tanaffos 2007; 6(4): 47-52)