Objective To study the value of serum carcinoembryonic antigen in monitoring the clinical effect of Gefitinib on treatment of advanced stage non-small cell lung cancer. Methods 36 cases of patients with advanced non small cell lung cancer were given Gefitinib orally 250mg/d, until disease progression or intolerance of side effects. The serum CEA levels in patients before and after treatment were determined. Results The clinical benefit response rate of Gefitinib treatment in patients with advanced non small cell lung cancer reached 77.8%. Among all the cases, there were 11 cases of PR, 17 cases of SD and 11 cases of PD. The total effective rate was 30.6% and disease control rate was 77.8%. Median survival time without tumor was 4 months, and median overall survival time was 9 months. The one year and two years survival rate was 38.9% and 22.2% respectively. One case even survived 30.3 months. The serum CEA levels in patients after treatment were significantly lower than that of before treatment. Conclusion The treatment effect of Gefitinib in advanced non small cell lung cancer effect is better, especially in women and adenocarcinoma patients. The serum CEA level could be used an index to monitor the treatment effect of Gefitinib in advanced non small cell lung cancer.