Theoretical and experimental endeavor were combined to investigate effects of Co-incorporating on crystal structure and oxygen vacancy formation energy and their influence on Sr2Co1 +Mo-x(1) (-) O-x(6 - delta) electrochemical performance systematically. From the X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), it is found that introducing Co decreases the ordering of B-site cations. Most importantly, Co-3 (+) content increases with the increasing Co, indicating more Co-O*-Co pairs are introduced within the compounds. Additionally note that the electrochemical performance of the materials is improved apparently when more Co is incorporated. Sr2Co1.5Mo0.5O6 - delta has an area-specific polarization resistance (ASR) as low as 0.030 Q cm(2) in a symmetrical cell at 800 degrees C. First-principles calculations found that the sequence of the oxygen vacancy formation energy was Co-O*-Co < Co-O*-Mo, indicating that Co-doping facilitates formation of oxygen vacancy, which coincides well with experimental results. Therefore, Co-incorporating in Sr2CoMoO6 is an efficient way to enhance electrochemical performance of cathode for IT-SOFCs.