Biodegradation is an effective way to degrade organic chemicals in a soil-wate-rmicrobes system.In order to account for the recalcitrance of residual chemicals in soil,a new theory-organic chemical sequestration inside soil particles has been applied in this paper.A mathematical model,which is developed based on this theory,can predict the time and extent of PAHs biodegradation.The model simulation results match successfully with the field data of a bioremediation process in LTU site on Alcoa's Massena,New York.By using mathematical model,3,4,5 and 6-ring-PAHs biodegradation processes and regularities in remediation sites have been predicted in this paper.