Abstract: Isotope dilution method was adopted in lab for the study of the effect of crude oil contamination on the rate of nitrogen mineralization and nitrification in soil. The results of the study showed that the organic nitrogen mineralization rate was not significantly affected by crude oil, while the nitrification process was delayed owing to the presence of crude oil. After 10 d of incubation, the number of ammoni oxidizing bacteria in soil with no crude oil been added increased from 1.6×10³ to 1.6×10⁶,while in the soil with crude oil added the number of ammoni oxidizing bacteria only increased to 1.6×10⁴. In another test, the number of soil ammoni oxidizing bacteria was 2.6×10⁶ before test, then adding crude oil, after 10 d of incubation the number of bacteria declined to only 10% of that ofthe original one.