Mixed conducting perovskite oxides are promising catalysts for high-temperature oxygen reduction reaction. Pristine SrCoO3-δ is a widely used parent oxide for the development of highly active mixed conductors. Doping a small amount of redox-inactive cation into the B site (Co site) of SrCoO3-δ has been applied as an effective way to improve physico-chemical properties and electrochemical performance. Most findings however are obtained only from experimental observations and no universal guidelines have been proposed. In this article, combined experimental and theoretical studies are conducted to obtain fundamental understanding of the effect of B-site doping concentration with redox-inactive cation (Sc) on the properties and performance of the perovskite oxides. The phase structure, electronic con-ductivity, defect chemistry, oxygen reduction kinetics, oxygen ion transport and electrochemical reactivity are experi-mentally characterized. In-depth analysis of doping level effect is also undertaken by first-principles calculations. Among the compositions, SrCo0.95Sc0.05O3-δ shows the best oxygen kinetics and corresponds to the minimum fraction of Sc for stabilization of the oxygen-vacancy-disordered structure. The results strongly support that B-site doping of SrCoO3-δ with a small amount of redox-inactive cation is an effective strategy towards the development of highly active mixed conducting perovskites for efficient solid oxide fuel cells and oxygen transport membranes.