Central composite design along with response surface methodology (RSM) was applied to improve the fermentation process in onion (Allium cepa) wine production. The effects of different fermentation parameters (time, temperature, and initial sugar content) were found to be significant with respect to the physicochemical and sensory properties of wine. The maximum score for the alcoholic content was obtained at $29.27^{circ}C$ fermentation temperature, 103.43 h fermentation time, and $27.52^{circ}Brix$ initial sugar content. The maximum score for overall palatability was obtained at $39.27^{circ}C$ fermentation temperature, 57.28 h fermentation time, and $22.14^{circ}Brix$ initial sugar content. The coefficients of determination ($R^2$) were 0.9620 and 0.9060 for alcoholic content and overall palatability, respectively. The ranges of the optimum fermentation conditions ($28{sim}32^{circ}C$, 80~90 hr, and $20{sim}25^{circ}Brix$) were obtained by superimposing the response surfaces with regard to the alcoholic content and overall palatability of onion wine.