Abstract: For meeting higher volumetric efficiency and minimizing pressure of trapped oil in external gear pump, the topic that whether the minimum distance between two relief grooves in small backlash could be applied in a large backlash or couldn't be proposed. Based on only considering two main leakages produced from the gap between backlash gap and relieving grooves, through trapped oil model created and verified in the early literature, two main leakages and it's volumetric ratios were calculated by simulation, and the curves of two volumetric ratios were respectively generated with different backlash values or speeds or output pressures. All the results indicated that the determination of no backlash or a backlash in external gear pump was meaningless. The volumetric efficiency was decreased with the increasing of the backlash value and output pressure. The trapped-oil phenomenon was more seriou with higher rotate speed and volumetric efficiency. The minimum distance between two relieving grooves in no backlash made trapped-oil phenomenon to be greatly improved, and the improvement made the volumetric efficiency higher. Finally, the minimum distance between the two relieving grooves in a small backlash can be applied in a large backlash, which provides a new reference in layout of relieving grooves for external gear pump.