It is considered that the urea injection DeNOx SCR(selective catalytic reduction) system is the only promising method to satisfy the worldwide NOx emission standards. As for the theoretical aspect, reactants of NO and NO2 with NH3 produce H2O, N2 and O2 which do not harm human beings and environmental as well. The realization of maximum NOx conversion (without using a post oxidation catalyst) is only possible with closed loop controlled urea dosing. It means built-in NH3 gas sensor have to be developed for detecting accurate NH3 concentration for the feedback system. Using YSZ(yttriastabilized zirconia) as a solid state electrolyte and In2O3 as a sensing material, this paper aims to study dependable NH3 gas sensor for the promising solution of DeNOx technology, which have a reproducible electric output signal, a high sensitivity and fast response.