Background: Obstructive lung disease is a growing health problem, especially in developed countries. This study aimed to compare Impulse Osillometry System (IOS) and Spirometry for evaluation of Chronic Obstructive Pulmonary Disease and asthma. Materials and Methods: The study groups contained 87 healthy people, 87 asthmatic patients and 56 COPD patients. Spirometry (FVC, FEV1, FEV1/FVC) and IOS (R5, R20, X5) measurements were performed for all the healthy subjects and patients. The results of IOS were compared with spirometric results. Results: Significant differences were detected among the 3 groups (control, COPD and asthma) in terms of all the spirometric parameters (FVC, FEV1, FEV1/FVC) and some parameters of IOS (R5,R20,X5) measurements (p <0.05). Among COPD patients, sensitivity for X5 (the best value of IOS measurements in the COPD group) was 76%. Also, in asthmatic patients, sensitivity for R20 (the best value of IOS measurements in the asthma group) was 77%. We found a correlation between R5, R20 and X5 with FEV1 in asthmatic patients, but only R5 had this correlation with FEV1 in COPD patients. Conclusion: We concluded that IOS can be an alternative for spirometry in the diagnosis of obstructive lung disease in patients with minimal cooperation. R5 can represent COPD severity. (Tanaffos2011; 10(1): 19-25)