BACKGROUND: Self inflating resuscitators are widely used for the patients who need positive pressure ventilation during transport. During self inflating resuscitator ventilation, the FiO2 may decreases even with the use of the oxygen reservoir. Such phenomenon may increase the difficulty and risk of transport of the critically ill patients. Twin self inflating resuscitator was assembled by being modified from two conventional ones to achieve more stable FiO2. METHODS: The twin self inflating resuscitator and one conventional adult self inflating resuscitator were tested on a test lung. In the twin resuscitator, two adult self inflating resuscitators were connected serially without distal unidirectional valve plate. The resuscitators were compressed at variable tidal volumes, respiratory rates and oxygen flow rates, guided by the monitor. RESULTS: With conventional self inflating resuscitator ventilation, the FiO2 was maintained over 95% until minute ventilation of 7,500, 1,4000, 17,500 ml at respectively 5, 10, 15 L/min oxygen flow rate. With serial type, the FiO2 started to decrease with the minute ventilation over 12,500, 24,000, 28,000 ml at 5, 10, 15 L/min oxygen flow rate. CONCLUSIONS: By simple connection of two self inflating resuscitators, the FiO2 during self inflating resuscitator ventilation could be maintained during almost two times of minute ventilation compared to conventional ones.