The marine diesel engines of Korean coast guard ships are typically high-power engines from MTU and Pielstick, in which the performance of lubricating oil is very important. As the coast guard ship engines experience numerous load fluctuations and their operating time increases, the properties of the lubricating oil, such as its kinematic viscosity, may change due to oxidation by heat and contamination by fuel and moisture. In this study, lubricating oil was collected from a coast guard ship by usage time (100, 200, 300 and 400 hr) and the amount of fuel contained in the sample was measured to analyze the degradation factors. Additionally, the kinematic viscosity was measured to analyze the degradation effect. Particularly, the oxidation stability, etc. were assessed to analyze the oxidation of the lubricating oils. As the lubricating oil usage time increased, the fuel mixing amount increased from 5.1% to 14%. Accordingly, the kinematic viscosity was degraded. However, with increase in the usage time of the lubricating oil, the oxidation due to heat and moisture mixing could not be confirmed.