Amplification of picosecond optical pulses by reflective semiconductor optical amplifier (RSOA) is theoretically investigated using a high-precision time-domain model. It is demonstrated that, compared to conventional SOA in similar conditions, pulses amplified by RSOA suffer from additional gain reduction of a few dB due to RSOA’s rear-facet reflection, which increases saturation level inside the device. The time delay and waveform of output pulse could be very different from RSOA or SOA providing the same small-signal gain. The model is then applied to a 40 Gb/s PCM signal to study the patterning effect in both SOA and RSOA.