Aiming at the condition monitoring problem of Diesel engine lubricating oil diluted by fuel, an experimental measuring apparatus was designed and completed to detect fluorescence intensity of oil samples by UV fluorometry. An UV LED with peak wavelength of 365 nm as emitting light source was chosen for exciting oil sample to produce fluorescence. Ultraviolet light from emitting light source passed through a 400 nm optical low pass filter and went into oil samples in quartz colorimetric utensil. Oil samples excited by ultraviolet light may produced fluorescent light. Firstly, fluorescent light was filtered by 400 nm optical high pass filter. And then filtered fluorescent light was transformed into electric signal by the photoelectric sensor with the detection wavelength range from 400 to 800 nm. After amplified by measurement circuit , digital multimeter can be used to detect the fluorescent light intensity. The signal amplification and measurement system were designed. The combination of optical high and low pass filters can reduce the interference of the ultraviolet light emitted by the ultraviolet light source to the fluorescence intensity of oil samples. Fluorescence intensity data of seven oil samples with different fuel concentrations of 20.3 Wt.%, 10.0 Wt.%, 5.0 Wt.%, 2.5 Wt.%, 1.5 Wt.%, 0.7 Wt.% and 0.0 Wt.% were obtained by the experimental apparatus as the above mentioned. The fitting equation of fuel concentration with fluorescence intensity was acquired. Finally, the oil sample of 7.5 Wt.% diesel oil content was used to verify the accuracy of the method. The fluorescence intensity of verifying oil sample was measured by the experimental apparatus. The ration of fuel to oil was calculated by fitting equation. The results showed that the ration of fuel to oil relative error between the calculated and the actual was 0.5%. The accurate measurement of the dilution of lubricating oil by fuel under the laboratory condition was realized.