This thesis aims to study the state estimation in 22-kV distribution system of Phitsamulok Provincial Electricity Authority to examine the distribution system in each phase independently and to propose the reduction technique of network size by using the theory of two-port network based on ABCD parameters of transmission line. The state estimator proposed is bases on the weighted least squares method which was developed on MATLAB program. The data obtained from Provincial Electricity Authority is simulated on PSS/Adept 5.0 program.