Substrate clogging is the worst operational problem for subsurface wastewater infiltration system ( SWIS ),nevertheless quantitative understanding of the clogging process is currently very limited.In this study,the developing process of clogging caused by organic particle accumulation and biofilm growth was investigated in two groups of lab-scale SWIS,which were fed with glucose ( dissolved organic matter) and starch ( particulate organic matter)influent and filied with the same substrate made of 50％ brown soil and cinder at a weight of 50％.Results showed that in glucose-fed systems the growth of biofilm in the substrate pores certainly caused remarkable reduction of effective porosity,especially for the high concentration organic wastewater,whereas its influence on infiltration rate was negligible.In comparison with biofilm growth,organic particles accumulation could rapidly reduce infiltration rate and the clogging occurred in the upper layer in starch-fed systems and the most important contribution of biofilm growth to clogging was accelerating the occurrence of clogging.