Based on the general mechanism of the coherent population oscillations, we propose the fundamental -harmonic fractional delay (FHFD) to evaluate the superluminal and slow light propagation in semiconductor optical amplifier (SOA). The sinusoidal and square-wave signals in SOA are investigated with the propagation equations. It is shown that the superluminal and slow light always accompany the signal distortion, and FHFD depends on the signal distortion as well as the incident power, the modulation frequency, and the optical gain.