It is well known that olfactory dysfunction relates to many etiologies such as obstructive nasal or sinus disease, upper respiratory tract infection, head trauma, aging, congenital anomaly, endocrine disease, irradiation or drug and so forth. Chronic paranasal sinusitis is the most common etiology in olfactory dysfunction, and Endoscopic sinus surgery can bring to improvement of olfactory function. A preoperative and postoperative evaluation of the olfactory function of 80 patients with chronic paranasal sinusitis was carried out using the quantitative test employing buthanol. Preoperatively normosmia was proven in 18 patients. Thirty-six patients were hyposmic, and 26 patients were anosmic. In the postoperative olfactory function test, thirty patients had normosmia, 35 patients experienced hyposmia, and 15 patients showed anosmia. Sixty-three percent of the patients with impaired olfactory function preoperatively had improvement after the surgery. And the most important factor in improvement of postoperative olfactory function was olfactory cleft patency.