Screening of diesel oil-degrading yeast strain isolated from mangrove rhizosphere in Tritih Kulon, Cilacap resulted 4 culturable yeast strains. Qualitative test was conducted to select the best yeast in degrading diesel oil. Qualitative test showed two best yeasts. For both two yeasts were conducted a quantitative test to determine in detail the process of degradation of diesel oil. These yeasts were Candida lusitanie and Cryptococcus laurentii. The detailed chemical composition of the residual oil was studied by gas-chromatographic techniques. Three highest of percentage area of hydrocarbon compounds were compared for assesing biodegradation potensial. Value of pH was measured for incubation time. Changes of peak were found in chromatogram, Candida lusitanie had better capability than Cryptococcus laurentii in comparison of peaks chromatogram. Hexadecane and methyl hexadecanoate had decreased by an average 90-95%, while the 9-octadecenoic acid, methyl ester decreased 30-40%. Mechanism of degradation had been seen by absorbing carbon molecules on hydrocarbon compound. Medium pH value ​​increased during incubation.