Marine diesel engine oil in service will be contaminated by water, fuels, coolants, and other degradation products such as oxidation, nitrification and sulfonation, which can also be produced under the influence of high temperature or combustion chamber gas. In serious cases, these contaminants and products can lead to the failure of equipment. Analyzing the used oils can reveal its condition, therefore determine the optimum oil changed period and fault source, and avoid the abnormal wear and corrosion of diesel engine during operation. At present, the traditional methods are as follows that the coulometric Karl Fischer titration be used to measure the water content in oils and gas chromatography to determine the fuel dilution. Due to long time and high cost for oil analysis, these two methods mentioned above are not widely used in monitoring oils. For analyzing the lubricating oil at the molecular level, Fourier infrared (FT-IR) spectroscopy can be used to monitor oil more effectively. Unfortunately, it is not yet widely used at present because of the complexity of the spectrum. The oil samples sum up to 20, which are contaminated by water, fuel, ethylene glycol coolant, high temperature oxidation as well as new oil. Water concentrations in oil are 0.11%, 0.22%, 0.44% and 0.88%. The durations of high temperature for oil oxidation are 299, 323, 371 and 395 h. The fuel dilutions of oil are 1.5%, 3%, 6% and 12%. Ethylene glycol coolant concentrations in oil are 0.1%, 0.2%, 0.4% and 0.8%. The oil samples were analyzed by the Agilent Cary 630 FT-IR with a factory set pathlength of 100 microns and with the spectral range of 4 000～650 cm-1. FT-IR spectra of all oil samples were obtained with FT-IR spectrometer. It is determined that the corresponding characteristic band ranges of water, oxidizing products, fuel dilutions and ethylene glycol coolant respectively are 3 150~3 500, 1 670~1 800, 745~755, 1 030~1 100 cm-1. Monitoring parameters of FT-IR spectra include center point, left boundary, right boundary, left baseline and right baseline. A quantitative analysis model for contaminants in used oil was established. The fitting equations are about the concentrations of contaminants and the peak area of FT-IR spectra. Correlation coefficients between the contaminants of water, fuel dilutions and ethylene glycol coolant and the corresponding peak area of FT-IR spectra are 0.977 9, 1.000 0 and 0.989 5, respectively. The correlation coefficient between oxidation time and the corresponding peak area of FT-IR spectra is 0.999 6. The maximum relative errors between the predicted from the fitting equations and the actual value are 10% for the content of water and glycol more than 0.2%, and are 1% for oxidation time and fuel dilution. By new oil proportional dilution, 3 routine used oil samples were monitored with FT-IR spectrum analysis. The results showed that the water content of one oil sample was 0.38%, which exceeded the standard; the another oil sample was diluted to 19%, which exceeded the standard, and the other oil sample was normal. In the case of oil samples, which exceeded the threshold of the water, the relative error of the FT-IR measurement is 4.6% when compared with the Karl Fischer method. The fuel dilution of the oil sample, which exceeded the requirement of the standard, was verified by the change of viscosity. The FT-IR spectrum analysis and the change of viscosity are consistent in judging whether the used oil is or not to be changed. FT-IR spectroscopy was used to analyze the lubricating oil in use, the peak absorbances was selected, and the area of peak absorbances was calculated. By means of the established fitting formula, the contaminants types and degrees of oil can be monitored quickly and reliably. FT-IR spectroscopy method can meet the engineering requirements of used oil monitoring to a certain extent.