Power line communications (PLC) presents an interesting and economical solution for automatic meter reading (AMR). If an AMR system via PLC is set in a power delivery system, a detection system for illegal electricity usage may be easily added in the existing PLC network. In the detection system, the second digitally energy meter chip is used and the value of energy is stored. The recorded energy is compared with the value at the main kilowatthour meter. In the case of the difference between two recorded energy data, an error signal is generated and transmitted via PLC network. The detector and control system is proposed. The architecture of the system and their critical components are given. The measurement results obtained from Turkey's power lines are given. This paper describes a prototype of the detector system for illegal electricity usage using the power lines. The target of this study is to discover new and possible solutions for the lack of literature concerning this problem.