Lab-scale composting of diesel-contaminated soil was carried out to evaluate the effects of organic amendments on the degradation of diesel oil, and identify the optimum mix ratio of compost with contaminated soil. The ratios of contaminated soil to these materials were 1:0.1, 1:0.3, 1:0.5, and 1:1 on wet weight basis. Experiments with only additives were carried out as a control, and biocide control experiments were also tested by the addition of mercuric chloride. The degradation rate of diesel oil was significantly enhanced by the addition of compost mainly due to the abundant available nutrient source and organic matter content. However, the excessive addition of these materials did not necessarily facilitate degradation rate. The most active degradation of TPH was observed in the mix ratio of 1:0.5, which showed a 98.4% degradation of TPH over 30 days. The degradation of n-alkanes was about two times higher than that of TPH. The cumulative loss of TPH also was not significantly changed with an increase in the amount of compost added. In the biocide control experiments, 10.7% of TPH removal of diesel oil occurred, while 99.6% of TPH was removed in the experiment without the addition of a biocide. This indicates that the inactivation of the microbial activity for degrading diesel oil was not complete although a significant suppression of microbial activity was observed. The carbon dioxide evolution rate, dehydrogenase activity and hydrogen utilization were well matched with the degradation of the diesel oil