The La0.6Sr0.4Co0.8Fe0.2O3−δ(LSCF)-Ce0.8Sm0.2O1.9(SDC) composite cathode for solid oxide fuel cells (SOFCs) based on SDC electrolyte, followed by being calcined at 1073 K, 1173 K, 1273 K, 1373 K and 1473 K, was prepared by infiltration of LSCF precursor solution into porous SDC scaffolds. The average LSCF particle size is about 50 nm, 0.1 μm, 0.15 μm, 0.2 μm and 1.2 μm respectively after being calcined at 1073 K, 1173 K, 1273 K, 1373 K and 1473 K for 4 h. The oxygen reduction reaction mechanism in the porous LSCF-SDC nano-sized composite cathode is studied through a comparison with that in the porous LSCF-SDC micro-size composite cathode. The oxygen reduction reaction mechanism in the LSCF-SDC composite cathodes is discussed in terms of frequency response, electrode resistance and reaction order at different oxygen partial pressures p(O2). Three elementary steps are considered to be involved in the cathode reaction: (ⅰ) Dissociation of molecular adsorbed oxygen into atom adsorbed oxygen on cathode surface; (ⅱ) Bulk diffusion of adsorbed of O2− in the bulk LSCF phase; (ⅲ) Oxygen ions transfer at the interface between the cathode and the electrolyte. For the cathodes sintered at 1073 K and 1173 K, the polar resistance in the medium frequency range was dominant and step (ⅱ) is the rate-determining step. The low frequency arc became obvious in the impedance spectra with increasing the calcination temperature of the LSCF precursors. The ratio of the polar resistance in the medium frequency range to the total cathode polar resistance decreases with increasing the calcination temperature, from 70.6% at 1073 K to 29.6% at 1473 K, but the ratio of the polar resistance in the low frequency range to the total cathode polar resistance increases with increasing the calcination temperature, from 4.9% at 1073 K to 58% at 1473 K, implying that the cathode process is limited by step (ⅲ) when the cathode calcination temperature is at 1473 K.