Atterberg limit tests were preformed on diesel oil contaminated soil and crude oil contaminated soil. The results show that Atterberg limits reduced with increasing of diesel oil content. When crude oil content changed from 0 to 8%, it has little effect on Atterberg limits. However, it rose from 8% to 16%, plastic limit slightly decreased, but liquid limit increased remarkably. A "pseudo-viscosity" caused by crude oil is the key factor for this phenomenon.