Abstract: The documentation of the frequency of occurrence of weed species that are prevalent in oil polluted areas is necessary to assess their capacity for tolerance and potential for phytoremediation of such polluted sites. The present study therefore investigated the impact of substrate amendment on the weed biodiversity of Waste Engine Oil (WEO)-polluted soil. Top soil (0-10 cm) was collected from an area of known soil seed bank on a farmland and measured into perforated bowls. The soil was then contaminated with WEO at 4 levels of pollution: 1.0, 2.5, 5.0 and 10.0% w/w WEO in soil. The unpolluted soil was the control. The entire set up was divided into two and then left for 5 months without mechanically disturbing the soil. The first set was amended with sawdust, whereas the second set left unamended for the remaining period of the experiment. After 10 months, there was general reduction in heavy metal composition of soil. Euphorbia heterophylla was the most prevalent weed, present in both amended and unamended soil treatments. Weed biodiversity studies showed that dominance indices ranged from 0.137-0.284 in the unamended soil treatments and 0.160-0.500 in the substrate-amended soil treatment levels. Substrate amendment of WEO-polluted soil therefore enhanced development of soil seed bank thereby improving weed diversity in the polluted soil probably necessitated by biodegradation of the soil contaminant.