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In this paper, a robust static state estimation method is proposed for radial distribution systems. Distribution automation is of main concern in distribution system operation. Static state estimation plays an important role to provide system operators with reliable data in the framework of distribution automation. In this paper, the state variables at the distribution substation are estimated with the L1-norm estimator that is robust to bad data. Once they are estimated, other state variables are evaluated by the nest structure of radial distribution systems. As the power flow calculation, the DistFlow method is used to evaluate them at the lower-stream node. The problem formulation is expressed as a constrained nonlinear optimization problem. This paper proposes an EPSO (Evolutionary Particle Swarm Optimization) based method for estimating the state variables at the distribution substation. The proposed method is successfully applied to the 69-node distribution system.