In this paper, a two-dimensional numerical study of a plasma density gradient effect on Farley-Buneman waves (FB waves) is per formed via a two-fluid code in which the electron inertia is neglected while the ion inertia is retained. We focused the simulations on the inter action between a single wave mode and the back ground E region where the vertical density gradient pro file and the weaker than FB thresh old ambient electric field were considered. From 2D density contour maps, it was found that the FB wave grows in the region of v_?¤Ne ¡__nv E > 0 (where v_?¤_nNe is the electron density gradient and v E is the electric field), the initial growth rate was in reason able agreement with the prediction of the combined linear theory of Farley-Buneman and gradient drift in stabilities, and the propagation speed was modulated by the gradient strength. Ac cording to the phase velocity evaluated by the Fourier anal y sis and peak to peak estimation method, the density gradients were found to have an effect of loing the phase velocity at saturation, which is smaller than ion-acoustic speed for large scale waves, and the results demonstrated that the reduction of phase velocity by a density gradient effect was larger for a longer wave length wave than shorter wave length curve. It was also found that the plausible density gradient effects seem to be related to the thickness of the density gradient region and vertical electric field where the FB wave was traveling. The thicker unstable layer would cause a greater phase velocity reduction than the thinner unstable layer might cause, and the large driving electric field would reduce the wavelength dependence of density gradient effect on the saturation phase velocity.