In this thesis, we studied the narrowband power line communication system. This system mainly utilizes the 60Hz power line as the medium to transmit network signals. In the beginning, we studied the power line channel characteristics and the coupling circuit structure, that was used to couple the signal to the power line. Impedance mismatch and signal attenuation may occur when the loading in the power line network changes. To this end we added a driver to the coupling circuit to reduce the output impedance, and hence enhance signal magnitude. In addition, we add the cross-phase coupling circuit with bandpass filter characteristics at the 220V socket. It was found that our cross-phase coupling circuit was able to improve the performance of the power line communication system when cross-phase transmission took place. Finally, we simulated the indoor power line network environment, measured several kinds of residential appliance noise and analyzed the influence on the power line communication system of the appliance noise.