Cation deficiency in perovskite oxides can alter the crystal structure, oxygen stoichiometry and other physi-ochemical properties, substantially promote the electro-catalytic activity for oxygen reduction reaction (ORR) at elevated temperatures. In this work, a small strontium deficiency was introduced into SrCo0.8Nb0.1Ta0.1O3-delta (S100CNT) to promote the electrochemical activity of ORR. Cation deficient Sr0.95Co0.8Nb0.1Ta0.1O3-delta (S095CNT) displayed higher oxygen vacancy concentration than that of stoichiometric S100CNT. The decreased average bonding energy (ABE) and the increased critical radius (r(c)) of S095CNT could be responsible for the decreased activation energy for oxygen ions transfer. Consequently, the ORR activity of S095CNT cathode was significantly improved. The S095CNT cathode shows an area specific resistance of 0.07 Omega cm(2) at 650 degrees C, which was only about 63% of S100CNT. Moreover, S095CNT displayed much higher CO2 tolerance than the state-ofthe-art Ba0.5Sr0.5Co0.8Fe0.2O3-delta (BSCF) cathode for SOFC. Thus, the fast ORR and high stability makes S095CNT promising material for devices involving oxygen electrochemistry such as solid oxide fuel cells, electrolysis cells, or gas separation membranes.