Abstract: In this paper the effect of particle size on the wear of hydraulic pumps was investigated. The test results show that the wear of the pump in a hydraulic system mainly depends on the relation between the size of abrasive particles and the dynamic clearance dimension of the key moving parts.The particles smaller than the dynamic clearance produce mild wear.The particles larger than the dynamic clearance do not produce wear. The particles near to the dynamic clearance dimension generate severe wear.The dynamic clearance dimension of the key moving parts in the pump can be used as a parameter to evaluate t he pump cont a-minant sensitivity and to determine the filter rating of the system. The formation mechanism of the wear debris of three body abrasive wear under liquid lubrication condition was analysed by means of a scanning electron microscope. To protect hydraulic pumps from abrasive wear, the system filter should be capable of removing particles near to the dynamic clearance dimension of the kev moving parts.