Hydraulic component is the basic composition unit of hydraulic systems and widely used. In the structure of a component, there exists a mass of fit clearances which are classified as planar structure, cone structure, cyclic structure and lineage structure, and etc. The rationality in science of fit clearances greatly affects the performance and the working life of a component. Researches are aimed at fit clearances of hydraulic component. Mechanisms of frictional wear and lubrication within clearance in luberication were analyzed. The impact of oil temperature change on the performance of hydraulic elements was studied by simulation. The research shows that with increasing of piston motion frequency, the oil temperature within clearance is rosen, the seal leakage is increased and hydraulic components volumetric efficiency is declined. At the same time, the relationship between initial temperature of oil film within clearance, oil temperature rise and leakage was analyzed. The conclusion is reached that the oil temperature and leakage within clearance increase with the initial temperature rising. The research results can be used by designers and users for reference.