Abstract: Aerobic composting is a commonly adopted bio-treatment technique to dispose solid organic wastes. Research on aerobic composting has been mainly on microbe screening, supplementary material arrangement,and dynamic changes of physical and chemical properties during composting. This paper monitored biochemical changes in the process of composting. The experiments were carried out in an indoor simulation system to better understand the mechanisms of aerobic composting through enzymology, and to provide criteria for evaluating maturity of compost and threshold of composting. The results showed that the dry matter of pig manure lost by about 30% in incipient stage, but lost only about 7% in intermediate and later stages though it was a long time during composting. During the composting the activities of main enzymes decreased continuously and gradually tended to be stabilized except the catalase. Polyphenoloxidase activity reached two peaks in the eleventh and the fiftieth day, and protease activity in the eighteenth and the sixty-fifth day during composting, respectively, indicating the biological changes in the course of decomposition and humification of organic carbon and organic nitrogen. The activity of invertase decreased by 95% and of urease and cellulase by more than 70% in the incipient stage of rapid decomposition, and finally reached a relatively lower level. This can be considered as a quantitative biochemical index that reflects the maturity of the compost.