Amplified spontaneous emission (ASE) in diode laser pumped double-clad fiber power amplifiers is studied experimentally. The dependences of ASE on fiber length and cross section of active core are discussed and the variations of ASE power as the function of pumping and signal power are investigated. There are indications that long fibers with large mode area need stronger input signals to suppress ASE. It is shown that a 150mW input signal can suppress the ASE by 40dB in a 4m large mode area fiber, while to efficiently suppress the ASE in a 10m fiber, stronger input signal is needed. 12 5W and 16 1W single frequency CW output power are obtained from 4m fiber and 10m fiber respectively. No stimulated Brillouin scattering (SBS) was observed.