Abstract: The governing equation is established regarding the effect of hydraulic transient on the contamination ratio of the cold-hot contaminated crude oil batch transportation due to reduced pipeline or elevation. FLUENT software is applied to simulate the cold-hot crude oil passing through the reduced pipeline where the effect of the transportation sequence and the velocity are investigated. The study shows that the contamination length grows faster in the reduced pipeline, while the length grows slower in the expanded pipeline. The contamination lengths are large for both cases of Russian Oil first and Daqing Oil first. And, the contamination ratio vibrates at the joint location of an expanded pipeline with regard to the cross-sections on upstream pipeline.