Grass swale is an effective measure for infiltrating, attenuating or conveying runoff generated from urban storm events. Several stormwater controlling measures have been constructed in an residential area of the Oriental Sun City, Beijing, China. Grass swale is one of the most basic measures used in this area. The effect of grass swale on controlling stormwater runoff and nonpoint source (NPS) pollution was observed during two storm events. The observed flow in the grass swale was much less than the calculated runoff without no measures, indicating the grass swale had an effect on attenuating runoff. Chemical oxygen demand (COD), total phosphorus (TP), and total nitrogen (TN) were selected as NPS pollution indicators. The observed COD, TP and TN concentration were 30-60, 15-20 and 0.1mg/l, indicating the water quality was still polluted. The rainfall-runoff processes were simulated using the Stormwater Management Model (SWMM) of USEPA. The result indicated that SWMM was able to model the general processes during the storm events. Besides, the codes of SWMM could be downloaded free, so it could be modified to adapt the local situation in China.