Background: Manual resuscitators are used to manually assist or provide positive pressure breaths during cardiopulmonary resuscitation, suctioning and intrahospital transport. However, the fraction of delivered oxygen (FDO₂) through these devices with different flow rates varies. Aim: To study the FDO₂ with manual resuscitator under different conditions. Methods: An adult manikin was intubated with 8 mm ID endotracheal tube and ventilated using an adult manual resuscitator. An oxygen analyser was connected between endotracheal tube and the manual resuscitator. The variables included with or without reservoir bag (RB), manual ventilation at 12 or 20 breaths/min and use of one or two hands while the FDO₂ was measured with oxygen flow rates at 2, 4, 6, 8, 10, 12 and 15 (L/min). Results: The maximum FDO₂ delivered was 95.3% and 97.6% with RB using one hand and two hands respectively. At . 10 L/min flow, FDO₂ was >90% with RB. At 2 . 4 L/min flow, the FDO₂ was <60% with RB and 25-40% without RB. FDO₂ was slightly higher with ventilation at 12 bpm than with 20 bpm. The effect of one- or two-handed ventilation was variable. Conclusion: With the use of a reservoir bag along with the manual resuscitator at an oxygen flow rate above 10 L/min, FDO₂ > 90% can be consistently delivered irrespective of single or both handed breath delivery or the ventilator rate. Without the use of reservoir bag, irrespective of other conditions, it is not possible to deliver FDO₂ > 60% with a manual resuscitator.