We have carried out some experiments to find out the behaviour of oil leakage from an O-ring packing used for the gasket and reciprocating parts. The chief points of the results of this investigation are : i) For the gasket : The seals can be made perfectly leakproof for oil pressure up to 280 kg/cm² (4000 psi limit of this test pressure). ii) For the reciprocating use : It has been shown theoretically and proved experimentally that the quantity of oil leakage from an O-ring is proportional to the square of the reciprocating velocity of the road, the n-th power of the kinematic viscosity of the oil. In this case we have n≒1 for the values of ν measured at the working pressure from analytical treatment but n=1.5 for ν measured at atmospheric pressure from experimental results.