Abstract: The gain characterization of photonic crystal fiber amplifier is theoretically and experimentally studied by using double-side pumped technique. The gain curves for different pumping power and different signal lights are obtained experimentally. When the power of the probe light is 6 W and the total pump power is 150.2 W, the output power, up to 72 W and the slope efficiency 60%, have been achieved eventually with double-side pumped. It is found in the experiment that when the total pump power is increased over a certain value, the parasitic oscillation and the self-pulsation are observed in the amplifier, because of the resonant cavity caused by the surface reflection of fiber and nonlinear effect, and in turn this degrades the system performance.