In this paper a computer model for automation in the design of the municipal stormwater drainage system is developed and rigorously verified. Although the program is tailored for application and use in Hong Kong, it can be readily adapted to other situations. The numerical model, which is designed for use on IBM/PC/XT or PC/AT microcomputers, is written in PASCAL language and is compiled by PC software TURBO PASCAL version 5·0 due to the various advantages of the integrated programming environment supported. The computer-aided design package for flow prediction and drainage design applies the Colebrook Formula and the Rational Method to route pipe flows through tree-type drainage networks. By using the computer package, drainage pipe diameters are adjusted automatically to fulfill the flow criteria, namely, that the pipe capacities are large enough to convey the peak runoffs and that the backwater levels are not higher than the finished levels and hence ensure that flooding will not be resulted at all locations. The monotonous and iterative conventional method for designing municipal stormwater drainage networks can be taken over by this model. During the execution of the user-friendly and easily operated computer program, adequate user oriented information will be furnished. Its verification is effected by application of the program for municipal stormwater drainage design in a typical example.