In order to study the effect of the oil aeration rate on bearing reliability, the minimum oil film thicknesses(MOFT) were measured by the change of oil aeration rate using the total capacitance method(TCM), and at the same time, to study the appropriateness as an index of bearing reliability, crankshaft temperatures were measured. The minimum oil film thicknesses were measured at each of five main bearings and No.1 connecting rod bearing. A scissor type linkage system was employed to measure the minimum oil film thickness of connecting rod bearing. Special techniques were devised to inject air into the oil, and to monitor the aeration rate by an on-line measurement system.The measured aeration rates are proportional to the oil pressure drop. The change of the MOFTs are very slight up to a 30% aeration rate in the test engine. But if the oil aeration rate exceeded 30%, the minimum oil film thicknesses were changed. So, it was found that the oil aeration rate should have a fixed limit in order to maintain bearing reliability.