Abstract: For purposes of calculating composite stiffness for dynamic model of gear pair in external gear pump, with the periodic changes of backlash location and meshing position, from definition of bulk modulus of elasticity, a model of calculating composite stiffness with contact stiffness and trapped-oil stiffness was derived, and then a simulation was executed in a trapped-oil cycle. The results showed that gear pair of external gear pump was different from normal gear pair; trapped-oil stiffness was a main factor in composite stiffness. It was concluded that trapped-oil made vibration slow down and the results can provide references for the future dynamic analysis of gear pair in external gear pump.