The objective of this research was an evaluation of amendments for supplementing organic matter for composting of diesel-contaminated soil. The materials used for this study were soil and sewage sludge, which was added as amendments for supplementing organic matter. The ratios of contaminated soil to these materials were 1 : 0.1, 1 : 0.3, 1 : 0.5, and 1 : 1 as wet weight basis. Experiments with only additives were carried out as control, and biocide control experiments were also tested by the addition of HgCl2. Degradation of diesel was affected by the sewage sludge mix ratio, and the most rapid degradation of TPH was observed in the mix ratios of 1 : 0.3 and 1 : 0.5 of contaminated soil to sewage sludge. However, excess addition of these materials did not necessarily facilitate degradation rate. Appropriate mix ratio for effective degradation was 1 : 0.5 as a wet weight, and 98.1% of TPH was degraded during the composting period. In biocide control experiments, 8.3% of TPH removal of diesel oil occurred, while 95.3% of TPH was removed in experiment without adding biocide. This indicated inactivation of microbial activity for degrading diesel oil was not completely occurring although significant suppress of microbial activity was observed. Carbon dioxide evolution rate and dehydrogenase activity were matched with the degradation of diesel oil well.