Running-in tests have been carried out for piston and cylinder in a piston pump and motor and the lubrication characteristics have been investigated. The friction characteristics on the sliding surfaces have been studied before the tests. The running-in tests have been done according to results of the friction tests. The running-in condition has been decided after considering parameters such as friction force, separation voltage, roughness, roundness and cylindricity. The results are summarized as follows: (1) The rate of running-in of the cylinder with low hardness and low stiffness is lower than that of the piston with high hardness and high stiffness. (2) Optimum taper angle at the piston top for friction force is found. (3) The friction force decreases with decreasing stiffness of the piston.