Stream structure in urbanized river network area has unique characteristics under the effect of physical factors and human modification. Hence the issues of what and how such changes worked on river network storage and flood control capacity become the focus of this study. Taking Shanghai as a sample area, which is one of the largest cities in China located in the eastern Yangtze Delta, by using Horton-Strahler classification and Horton laws as reference based on the stream classification system that is commonly adopted in Shanghai and other cities around it, this paper analyzed the stream structure characteristics in each catchment under various urbanization levels;approached the effect of urbanization on stream structure development; and demo nstrated the possible relations between stream structure and river network storage and flood control capacity. The flood storage and control functions of streams in each order were further discussed in this paper. The main results are: (1 ) The stream number and length within the river network were highly developed in the study area. Stream number developed better than stream length. (2) The stream structure could only be modified when urbanization was up to a certain high level. Physical laws still played important roles in those catchments with lower urbanization level. (3) The stream structure expressed the possible trends from comprehension to simplicity, from multiform to singleness during the process of urbanization. (4) There was an obvious converse change between river network storage and flood control capacity and urbanization level. River network stor age and flood control capacity was influenced both by the quantity of water area and stream structure and much more closely related to the number and length of streams in the lower order. (5) For those streams in the higher order, the stor age capacity was stronger than the control capacity and the converse situatione xisted in those streams in the lower order. (6) proper quantity of water area and better stream structure were the infrastructures to assure ecological flood storage and control in urban area.