An analysis of green roof water quality was done to answer this question: does the use of green roofs reduce or contribute to storm water pollution? Green Roofs can increase the overall water retention of an urban area and lower the volume of expected stormwater runoff. A Green Roof (GR) prototype was used to compare the runoff contamination level with a Non- Green Roof (NGR) system. Two tests were performed: the first one using rainwater samples collected over a span of three months and the second from a synthetic rainwater sample created in a laboratory. The sample set using rainwater source had a significant increase of Phosphate and a small decrease in Nitrate concentration within the GR system compared to the NGR system. The sample set of synthetic rainwater source had a larger reduction of Nitrate compared to the accumulation of Phosphate. The experiment also includes a peak flow calculation of GR and NGR systems within a study area in the City of Fullerton. GR and NGR systems were simulated using a hydrologic modeling software, Storm Water Management Model (SWMM). A 25%GR, 50%GR, and 60%GR runoff was compared to the NGR. All three GR systems helped reduced the runoff flow and depth. Furthermore, the proposed conditions were significantly effective in decreasing max flow, max full flow, and max full depth. Both rain water samples and synthetic rain samples show that GR system can improve water quality by decreasing nitrate concentration compared to the NGR system.