Cigarettes, the most popular method of smoking, consist of finely shredded tobacco rolled in light-weight paper. Smoke from the average cigarette contains around four thousand chemicals, some of which are highly toxic and at least 43 of which cause cancer. Nicotine, a major constituent of tobacco smoke is both poisonous and highly addictive1. FEF25- 75 %(L/sec) is the average flow rate over the middle of vital capacity. This test indicates the patency of smaller airways less than 2 mm in diameter. Aim of this study was to estimate effect of smoking on Forced expiratory flow during 25-75% of expiration (FEF25-75%) in asymptomatic smokers. The study was designed as cross section study which included 449 subjects of which 224 were in group 1 ( 0-10 Pack Year), 139 were in group2 (11-20 Pack Years), and 87 were in group 3 ( 21- 30 Pack Years ). The mean value of FEF 25-75% POST in group 1 (0-10pack years) is 3.197+1.153, Group 2 (11-20 pack years) is 2.397+1.162, and Group 3 (21-30 pack years) is 1.368+0.663 respectively. Smoking causes significant deterioration of lung function which can easily detected by using a spirometer. FEF25-75 % (L/sec) is a simple test that could be useful in early diagnosis of peripheral airway obstruction and its treatment, thereby improving the quality of life.