The performance of grass filter strips (GFS) in abating $PO_4$-P concentrations from the forage cropland was tested in an experiment on the 10% slope in Grassland and Forages Division, National Institute of Animal Science, Rural Development Administration (RDA) from October 2007 to September 2009. Forage croplands with rye-corn double cropping system applied with chemical fertilizer and livestock manure (LM) were compared in a natural condition. The plots were hydrologically isolated Main plots consisted of the length of GFS, such as 0m, 5m, 10m and 15m. Sub plots consisted of the type of LM, such as chemical fertilizer (CF), composted cattle manure (CCM) and composted swine manure (CSM). Concentrations of PO4-P in surface runoff water were reduced as the length of GFS increased. Especially, GFS with 10m and 15m reduced $PO_4$-P concentrations significantly compared to that with 0m and 5m (p<0.05). The results from this study suggest that GFS improved the removal and trapping $PO_4$-P from forage croplands.