Due to the recent growth of the Internet and the rapid development of information and communications technology, the number of data centers and the amount of power consumed are expected to increase rapidly. However, these data centers consume large amounts of electric power throughout the year, regardless of working days or holidays; hence, measures to save energy at these facilities are urgently required. A numerical analysis was conducted in this study by changing various variables to examine the effects of various design and operating conditions on outdoor air-cooling systems and to derive some operating plans to minimize electric power consumption through the introduction of outside air into the computer room. When designing the system, it is desirable to select airflow considering various factors, such as the heat generated by the computer equipment, the efficiency of the fan, and the performance of the refrigerator, rather than the theoretical maximum outdoor airflow. As a result, the optimal design air volume was calculated according to the equipment load. Consequently, the optimal design air volume of the modeled computer system was obtained as 26,200 m(3)/h. The application of this optimal design air volume is expected to yield an annual energy-saving effect of 56.1% compared with the power consumption in the air-conditioning period during which the application of outdoor air cooling is impossible.