This paper have examined the optimum combination of SNCR and SCR by varying SNCR injection temperature and NSR ratio along with SCR space velocity. NOx reduction experiments using a SNCR/SCR combined process have been conducted in simple NO/NH3/O2 gas mixtures. Total gas flow rate was kept constant 4 liter/min throughout the SNCR and SCR reactors, where initial NOx concentration was 500 ppm in the presence of 5% O2. Commercial catalyst, sulfated V2O5-WO3/TiO2, was used for SCR NOx reduction. The residence time and space velocity were around 1.67 sec, 2,400 h-1 and 6,000 h-1 in the SNCR and SCR reactors, respectively. SNCR NOx reduction effectively occurred in a temperature window of 900～950℃. About 88% NOx reduction was achieved with an optimum temperature of 950℃ and NSR=1.5. SCR NOx reduction using commercial V2O5-WO3-SO4/TiO2 catalyst occurred in a temperature window of 200～450℃. 80～98% NOx reduction was possible with SV=2400 h-1 and a molar ratio of 1.0～2.0. A SNCR/SCR(SV=6000 h-1) combined process has shown same NOx reduction compared with a stand-alone SCR(SV=2400 h-1) unit process of 98% NOx reduction. The NH3-based chemical could routinely achieve SNCR/SCR combined process total NOx reductions of 98% with less than 5 ppm NH3 slip at NSR ranging from about 1.5 to 2.0, SNCR temperature of 900℃～950℃, and SCR space velocity of 6000 h-1. Particularly, more than 98% NOx reduction was possible using the combined process under the conditions of TSNCR=950℃, TSCR=350℃, 5% O2, SV=6000 h-1 and NH3/NOx=1.5. A catalyst volume was about three times reduced by SNCR/SCR combined process compared with SCR process under the same controlled conditions.