Abstract: A field experiment was conducted to study the effects of agricultural waste cattle dung on the bio-disposal of oily sludge by composting the cattle dung with oily sludge at a ratio of 1:1 in small (1 m×0.5 m×0.35 m) and large (3 m×3 m×0.35 m) piles. During the 115 days experiment of composting, the C/N ratio, total petroleum hydrocarbon (TPH) content, TPH fractions, bio-toxicity, and microbial diversity of the compost were analyzed. The initial C/N ratio of the compost was 26.4. With the increasing time of composting, the C/N ratio of the compost in small and large piles decreased to 18.4 and 18.5, respectively, suggesting that the composed material was matured. On the 115 day of composting, the degradation rate of the TPH in small compost pile reached 22.96%, being higher than that in large compost pile. Throughout the composting process, the saturated fraction of the TPH presented a decreasing trend, and the aromatic hydrocarbons in small composting pile decreased after an initial increase while those in large compost file showed an increasing trend. The relative germination index (GI) of wheat seeds indicated that the compost at the beginning of composting had a higher phytotoxicity, which was decreased during composting, and the two piles got matured by the end of composting. The detection with denaturing gradient gel electrophoresis (DGGE) showed that with the increasing time of composting, the microbial community structure of the compost also had a greater change. It was suggested that the differences in the TPH degradation between small and large composting piles could be resulted from the difference of the microbial community in different volumes of composting piles.