The test method for diesel fuel diluent in used diesel engine oils by gas chromatography was discussed. And the impact of injection volume and integration time of chromatography peak of diesel oil on the final result was emphatically studied. According to the regression coefficient R2 of standard working curve, the optimum values of injection volume and integration time were determined and discussed finally. The repeatability and reproducibility of the method were also examined. The results showed that the precision of this test method is high, the repeatability is less than 0.3% and the reproducibility is less than 1.6%. The test method can meet the requirements of the determination of diesel fuel diluent in used diesel engine oils on the equipment maintenance and oil research application.