The conventional design method of the storm sewer network has two critical problems, i. e., the manhole energy loss is not considered in the design code, and the reasonable design is not easily obtained satisfying many kinds of design conditions. This paper presents an optimal design method of the storm sewer network considering with the manhole energy loss. The manhole and pipe diameters are determined by the optimality criteria method based on the minimization of the friction loss of pipe and manhole energy loss while the pipe slope and other conditions are known. Then the pipe slope is determined by the genetic algorithm based on the minimization of a cost performance function.