The double perovskite oxides La2-xNiMnO6-delta (x = 0.0, 0.1, 0.2, 0.3) are evaluated as potential cathodes for intermediate temperature solid oxide fuel cells. The effects of La-deficiency on the crystal structure, high temperature thermal and electrical properties, and catalytic activity for oxygen reduction reaction are comprehensively investigated. La2-xNiMnO6-delta crystallizes in monoclinic structure with the space group P2(1)/n. The cell volume expands with the increase of La-deficiency. The highest electrical conductivity of 25 S cm(-1) is found for La1.8NiMnO5.71 at 800 degrees C in air. Introducing La3+ deficiency greatly improves the amount of chemical adsorbed oxygen species on material surface, which promotes the oxygen reduction reaction of La2-xNiMnO6-delta cathode. The optimum composition, La1.8NiMnO5.71 exhibits the lowest area specific resistance of 0.21 Omega cm(2) at 700 degrees C in air. Meanwhile the peak power density of the anode-supported single cell with La1.8NiMnO5.71 cathode reaches 0.6 W cm(-2) at 700 degrees C. The electrochemical impedance spectra measurements combining with distribution of relaxation times analysis prove that the charge transfer process is the rate-limiting step of the ORR reaction.