Engine lubricating oil perform functions including wear reduction, friction reduction, piston cooling, corrosion prevention, cleaning pistons, preventing leakage and serving as a hydraulic media. Oil aeration is the entrapment of air into engine oil during operation. Aeration would affect oil density, viscosity and its sound velocity, with a detriment to such properties as lubricity, cooling and lubricating temperature, possibly resulting in worse engine working environment. In this paper, a new volume method with temperature compensation is introduced and proved to be indispensable. The measurement of oil aeration rate is performed with the main oil gallery of a four cylinder, turbocharged, high-speed diesel engine under different operating conditions. The temperature compensation is carried out for the measured oil aeration rate and the compensation effect evaluated. The variation of oil aeration rate with time after oil leaving the main oil gallery is also presented. The measured oil aeration rate is somewhat greater than normalized one, while the latter is much greater than on-line oil aeration rate in the main oil gallery. After oil drainage from main oil gallery, the oil aeration rate and oil temperature decrease steeply first and then gently to zero and ambient temperature, individually. The normalized oil aeration rate increases approximately linearly with engine speed, but hardly varies with engine load.