In order to study the effects of plastic bulking agents on processing of swine manure composting, three parallel composting experiments were conducted with laboratory scale composting reactors. Same amount of swine manure was mixed with equal volume of wheat straw or plastic bulking agents. Temperature, oxygen concentration, moisture content, volatile solid content, soluble carbohydrate content, cellulose content, crude fat content and crude protein content were monitored and recorded respectively. Mathematical models were employed to detect changing of free space and its effect on composting substrate decomposition. According to the results of composting experiments, temperature of composting pile using wheat straw as bulking agents raised faster than the one with plastic bulking agents at the beginning of the composting. Meanwhile wheat straw bulking agents showed better performance on water holding capacity. However, plastic bulking agents was better at keeping free air space of the whole composting pile.