In peatlands, nutrients often change qualitatively and quantitatively through the various biogeochemical processes such as the retention by sedimentation and plant uptake and the transformation from inorganic to organic and from dissolved to gaseous. The nutrient cycling function of peatlands plays an important role in the development of water quality in the downstream. The function varies according to the external nutrients load as well as vegetation and hydrological condition of the peatland. Therefore, quantitative information on the function under various nutrient-loaded conditions is required to utilize the function in the effective nutrient management of river basins. However, few studies have examined the nutrient cycling function of Japanese peatlands; none has revealed the effects of artificial nutrients load on the function. The objectives of this study was to evaluate nutrient retention quantitatively of a poor fen under relatively natural condition located in the Yufutsu Plain, Hokkaido. Furthermore, by comparing with a nutrient-loaded poor fen in the same region, we examined the effects of nutrients load on the retention function.