The characteristics of moisture ventilation in a dry room were studied numerically The effect of three important parameters: position of outlets, aspect ratio($eta$) of horizontal plane and air exchange rate(N), was analyzed by using the scales of ventilation efficiency. The ventilation performance was evaluated by varying the aspect ratio and air exchange rate in the four types of outlet position. It was shown that the ventilation performance was improved by decreasing the aspect ratio in the longitudinal arrangement of outlet. The highest ventilation performance was determined when $eta$ was 4 in the transverse arrangement of outlet. Regardless of the aspect ratio, the ceiling arrangement of outlet played more dominant effect on the ventilation efficiency than the floor arrangement. In every type and aspect ratio, the increase of air exchange rate to improve ventilation performance was appropriate up to N=60 /h.