A new conception of effective gain recovery time of semiconductor optical amplifier (SOA) was brought forward, and a simple analytical expression for the gain r ecovery time was derived. Factors that affect gain recovery time, such as bias c urrent, probe power and device length were analyzed respectively on the base is of application of SOA to the cross gain modulation wavelength conversion. Gain r ecovery time was measured experimentally. It is shown that the experimental resu lt agrees well with the theoretical calculation.