On the basis of field surveys of stormwater runoff conducted in urban roadways, stormwater runoff processes were characterized to determine their loading effects on the receiving water body. The main results are as follows: (1) In stormwater runoff, 82-96 % of the total rainfall amount was discharged owing to hydrologic abstraction. (2) The percentages of the particulate portion were 96, 60-70 and 26 % for phosphorus, organic matter (chemical oxygen demand and total organic carbon), and nitrogen, respectively. (3) The pollution load carried by automobiles and the remaining load from previous rainfall affect stormwater runoff. (4) A cumulative flow of 2-5mm, averagely 3.5mm, was considered to be the most effective loading cutoff in the first flush runoff. (5) Total stormwater runoff could be decreased by 50 % in a year using these first flush runoff cutoffs.