The bio-rack is a new approach for treating low-concentration polluted river water in wetland systems. A comparative study of the efficiency of contaminant removal between four plant species in bio-rack wetlands and between a bio-rack system and control system was conducted on a small-scale (500 mm length x 400 mm width x 400 mm height) to evaluate the decontamination effects of four different wetland plants. There was generally a significant difference in the removal of total nitrogen (TN), ammonia nitrogen (NH3-N) and total phosphorus (TP), but no significant difference in the removal of permanganate index (COD(Mn)) between the bio-rack wetland and control system. Bio-rack wetland planted with Thalia dealbata had higher nutrient removal rates than wetlands planted with other species. Plant fine-root (root diameter < or = 3 mm) biomass rather than total plant biomass was related to nutrient removal efficiency. The study suggested that the nutrient removal rates are influenced by plant species, and high fine-root biomass is an important factor in selecting highly effective wetland plants for a bio-rack system. According to the mass balance, the TN and TP removal were in the range of 61.03-73.27 g/m2 and 4.14-5.20 g/m2 in four bio-rack wetlands during the whole operational period. The N and P removal by plant uptake constituted 34.9%-43.81% of the mass N removal and 62.05%-74.81% of the mass P removal. The study showed that the nitrification/denitrification process and plant uptake process are major removal pathways for TN, while plant uptake is an effective removal pathway for TP.