La-Co-O composites with varied La/Co molar ratios (defined as LCO-r) were synthesized as catalysts for CO oxidation and CO + NO reduction reactions. XRD, N2 physisorption, H2-TPR, O2-TPD and XPS measurements were used to characterize the physicochemical properties of the catalysts, which showed that the increase of r value leads to varied phase structures, surface area, oxidizing ability, thermal stability and surface atomic exposures. Catalytic tests showed that LCO-0.3 is the best catalyst for CO oxidation and LCO-0.5 is the best for CO + NO reduction, suggesting the crucial factors controlling the reaction rate was changed, due to the different properties of O2 and NO. Nevertheless, the co-existence of Co3O4 and LaCoO3 is important for both reactions, as synergistic effects facilitating the activation of CO and O2 (or NO) were induced between them. A significant decrease in the activity of both reactions was observed at r > 1.0, due to the formation of chemically stable La2O3, which covers the catalyst surface and restrains the reaction.