Soil biofiltration is an environmentally-sound technology for elimination of VOCs, odorous and NOx compounds from a low concentration, high volume waste gas streams because of its simplicity and cost- effectiveness. This study investigated the optimal mixture fraction of briquet ash, compost, soil and loess for NOx degradation. Extreme vertices design was used to examine the role of four components on NOx degradation. Under our experimental conditions, 74.5% of NOx degradation was observed, using a model mixture(25% briquet ash, 10% compost, 30% soil and 40% loess) containing 100 ppb of NO. It was shown that experimental design analysis could allow selecting optimal conditions in such biodegradation processes in this study.