State Estimator is the basic functionality necessary for the “smart” operation of power networks, both in transmission and distribution side. Being integrated in a power management system and facing different quality of network data, robustness and efficiency are essential to provide accurate estimation of a power network operation state. Distribution State Estimator, applied in Distribution Management System development project in Guizhou Province in China, was tested in field operation. Estimation principles and test results are described in this article. (4 pages)