Abstract: In order to understand further the roles that bacteria plays in constructed wetlands, the vertical flow microcosms were established in the current study, and six treatment gradients of sulfate streptomycin were applied into microcosms (0, 1, 1.5, 2.0, 2.5 and 3.0 mg/kg sand) to investigated the relationships between the bacterial removal and enzyme activities or and compound transformations. Results indicated that CMC-cellulase, -glucosidase and protease activities did not change with the bacterial gradual removal (P 0.05), but urease, nitrate reductase and acid phosphatase activities significantly decreased (P 0.05). Also it was more interesting that invertase activity significantly increased with the bacterial gradual removal (P 0.05). Among six biochemical processes, except for cellulose decomposition and inorganic phosphorus transformation did not change with the bacterial gradual removal (P 0.05), the remaining four biochemical processes such as ammonification, nitrification, denitrification and organic phosphorus decomposition significantly decreased (P 0.05). Principal components analyses showed that the total change in compound transformations clearly distinguished from the microcosms treated with streptomycin (P 0.05), but the total change in enzyme activities did not.