Objective: To determine the amount of microorganisms presenting in the operating room air at the time before, during and after surgery, and the pollution source of the operating room air. Subject and method: 300 air samples taken in 15 operating rooms by AirPort MD8 machine of Sartorius company (Germany). "the counting techniques, culture, identification of bacteria, fungi were performed in the microbiology department. Result Before surgery, all of the hospital's operating rooms are in the range of microorganism standard in the air " 180 du/m3). At the time of surgery, the amount of microorganisms initially increased in all operating rooms with range from 80.2 to 235.4 du/m3, and 20 percent of the operating rooms with microorganisms exceeded 180 du/m3. After the first surgery and the last surgery of one day the amount of bacteria, fungi in the operating rooms are greatly increased in comparison with before surgery. The amount of microorganisms in the operating room air over 180 cfu/m3 increased to 62 percent, and 133 percent when one and more than 6 persons additionally entered the operating room, respectively. The amount of bacteria in the operating room air is closely related to times of opening and closing doors (p0.05).