In narrow band power-line communication (PLC) systems, which use frequency band below a few hundred kHz, the noise on power-line is non-white and non-stationary. Under such environment, the performance of Orthogonal Frequency Division Multiplex (OFDM) modulation system is analyzed, and time and frequency dependence of bit error rate (BER) is clarified. In addition, the possibility of performance improvement with the symbol level repetition coding employing cyclo-stationary feature of power-line noise is presented.