Abstract: In this paper,we compare microwave FEL numerical simulations using our code named WAGFEL which is a three-dimensional code,with the experimental data of Livermore Laboratory.WAGFEL evolves the electrons'energies and ponderomotive phase according to the averaged single-particle equations derived by KMR.and the fields according to the paraxial wave equation with the source term as derived byColson.The particle motion is fully three-dimensional.The field solver is two-dimensional,because only the lowest TE01 mode is considered in the rectangular waveguide.In order to obtain good agreement between experiment and theory,the code must include the effect of the longitudinal space-charge force in the KMR equations,the effect of v2<c (v2 is the longitudinal velocity of electron in the wiggler).and the effect of the shift in signal phase velocity due to the presence of the waveguide.The comparision includes microwave power output as a function of wiggler magnetic field for 1,2,and 3m long wigglers microwave power output as a function of wiggler length:the phase of TE₀₁ mode as a function of wiggler length:the output power as a function of input microwave power:and output power as a function of wiggler length for tapered wiggler.Comparision between numerical simulations using WAGFFL and the experimental data of Livermore shows that the experimental detuning curves for 1,2,and 3m long wigglers agree very well with our simulations except for wiggler magnetic field Bw which is 8% higher in the simulation.WAGFFL uses all the experimental data without free parrameters.The experimental postsaturation oscillation in power,named synchrotron oscillation,is in agreement with our simulation,but has a difference of about 25% from the analytical value.Through the above comparision it can be concluded that our numerical simulation by WAGFFL is reasonable.Therefore.WAGFFL code can be used to design microwave FEL.