This study evaluated the use of humic acid (HA) to enhance biodegradation of petroleum hydrocarbons in a soil contaminated with diesel oil at an initial total petroleum hydrocarbons (TPH) concentration of 5000 mg kg(-1) soil (0.5%). The HA was characterized using Fourier transform infrared (FTIR) spectroscopy. Six pans of 0.5% diesel oil-contaminated soil were preparedthree pans were supplemented with three rates of HA (10, 20, and 30g HA kg(-1) soil) and urea, one pan was supplemented with only urea, another pan was without any amendment (control), and the sixth pan was supplemented with a diesel-oil-contaminated soil that was steam-sterilized (nonbiologically active soil) . These pans were used to investigate the effects of HA on the biodegradation of TPHs and the population of hydrocarbon-utilizing bacteria (HUB) by measuring residual concentration of TPH and microbial count, respectively, during a 75-day incubation experiment. At the end of the incubation, the lowest reduction in TPH concentration was observed in the sterilized soil. The TPH in HA amended pans had reduced from 55.55 to 80.70%, while other biologically active soils recorded reductions between 21.39 and 32.59%. The pan containing only nutrient recorded lower concentration of TPH (32.59%) compared to the control (21.39%). The HA-amended pans also recorded highest count of HUB that ranged between 3.6 x 10(6) and 11.5 x 10(6) cfu g(-1). The pan containing only urea recorded HUB count of 5.9 x 10(6) cfu g(-1), while the control had HUB count of 2.2 x 10(6) cfu g(-1). The results of this study showed that HAs enhanced biodegradation of petroleum hydrocarbons and also stimulated the proliferation of soil bacteria.