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/NH$_3$/O$_2$ 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% O$_2$. Commercial catalyst, sulfated V$_2$O$\_$5/-WO$_3$/TiO$_2$, was used for SCR NOx reduction. The residence time and space velocity were around 1.67 sec, 2,400 h$^$-1/ and 6,000 h$^$-1/ in the SNCR and SCR reactors, respectively. SNCR NOx reduction effectively occurred in a temperature window of 900-950$^{circ}C$. About 88% NOx reduction was achieved with an optimum temperature of 950$^{circ}C$ and NSR=1.5. SCR NOx reduction using commercial V$_2$O$\_$5/-WO$_3$-SO$_4$/TiO$_2$ catalyst occurred in a temperature window of 200-450$^{circ}C$ 80-98% NOxreduction 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 NH$_3$-based chemical could routinely achieve SNCR/SCR combined process total NOx reductions of 98% with less than 5 ppm NH$_3$ slip at NSR ranging from about 1.5 to 2.0, SNCR temperature of 900$^{circ}C$-950$^{circ}C$, and SCR space velocity of 6000 h$^$-1/. Particularly, more than 98% NOx reduction was possible using the combined process under the conditions of T$\_$SNCR/=950$^{circ}C$, T$\_$SCR/=350$^{circ}C$, 5% O$_2$, SV=6000 h$^$-1/ and NH$_3$/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.