A microwave free electron laser which consists of Marx generator, vacuum diode, bifilar helical wiggler, and guide solenoid was designed and constructed. The analysis of the magnetic field distributions of the bifilar helical wiggler and computer simulation of electron trajectories with the perpendicular effect of space charges led to the conclusion that the magnetic field distributions are suitable for the electron beam injection. Output frequency in a single <TEX>$TE_{11}$</TEX. mode derived from the dispersion relation with parameters of 300G wiggler field, 4.4 kG guide field, and 170 keV electron beam energy was about 11 GHz. And this agreed with the frequency range of 9.78 GHz~11.7 GHz deduced from the experimental result. The gain was calculated by considering the effect due to the axial spread of the electron velocity distribution.