A 10 Gbit/s OFDM upstream transmission has been investigated by using a Fabry-Perot laser diode (FP-LD) based ONU with seeding-light in a colourless WDM-PON. Also demonstrated is the relationship of CW injection power and SNR of each OFDM subcarrier for a modelocked FP-LD. Moreover, when a 16-QAM OFDM modulation is applied in a modelocked FP-LD, -15.7 dBm received sensitivity at the BER of 3.8 x 10(-3) can be obtained after 20 km fibre transmission.