Microcosms were employed to study the effect of polychlorinated biphenyls (PCBs) in vegetated (with Kandelia obovata seedlings) and non-vegetated sediment on microbial and enzyme activities. The sediment was treated with 180, 900, 1800, 2700μg•kg-1 of 2,2’,4,4’- tetrachlorobiphenyl and 2,2’,4,4’,6,6’-hexachlorobiphenyl. Both vegetated and non-vegetated sediment were sampled at 1, 30, 90, 180 d and 270 d respectively following planting. Microbial enumeration, intensity of sediment respiration and microbial diversity were determined. Polyphenol oxidase and peroxidase activity were also analyzed. Results demonstrated that vegetation increased the amount of sediment bacteria and actinomycetes rather than fungi. PCB addition increased the sediment bacteria and actinomycetes population. The amount of bacteria in non-vegetated sediments was related significantly to concentrations of PCBs at day 30 and 90 with an r value of 0.974 and 0.942 respectively. Microbial respiration intensity paralleled microbial populations over course of experiments; increasing in the present of PCBs. Bacteria was the predominate microorganism population in sediment, and the determinant of microbial diversity. Microbial diversity of vegetated sediment were lower than that in non-vegetated sediment. In contrast, there were no significant differences in microbial diversities between PCBs treatments and the control. Polyphenol oxidase produced by sediment microorganism were stimulated by PCB addition. However, peroxidase was initially inhibited at the beginning of the experiment but increased with time. Results indicated that these two enzymes are sensitive to PCBs contamination.