This paper presents treatment efficiency and plant growth of a subsurface-flow constructed wetland system (23 m in length, 6.5 m in width, 0.65 m in depth) over one year after its establishment on floodplain of a stream in June 2000. An upper layer of 10 cm in depth was filled with course sand and the main biological layer of 50 cm depth with crushed stone with 8 - 15 mm in diameter. The system was planted with common reeds (Phragmites australis) grown on pots. Effluent discharged from a secondary-level treatment plant was funneled into it. Reed stems emerging in April 2001 grew up to 145.9cm until July 2001. The number of reed stems in July 2001 increased by about 11 times compared with that just after planting. The system was inundated seven times by storms over the monitoring period. Reeds were slightly bent after flooding, however they returned to almost upright standing in a couple of weeks. Small portion of inside slope of berm was eroded and the system surface had a sedimentation of 2 - 3 mm in depth. The average removal rates for SS, $BOD_5$, T-N and T-P was 73%, 70%, 53%, and 72%, respectively. The purification efficiencies for SS and $BOD_5$ were fairly good. The reduction rates for T-N was relatively low for the period of late fall through winter until early spring due to lower water temperature which retarded microbial nitrification and denitrification mechanisms. Reduction in the concentration of T-P during fall and winter was relatively higher than that during spring. Leach of phosphorous from plant litters lying on system surface and slight resuspension of precipitated phosphorous in substrates resulted in lower reduction for T-P in spring.