In this work, structural and catalytic properties of LaCo1-yMgyO3 and La1-xCaxCo0.50Mg0.50O3 nanocatalysts with x=0.00, 0.15, 0.30, 0.45, 0.60, 0.75 and y=0.00, 0.25, 0.50 prepared by citrate method are investigated. The structural characterization using X'Pert package and Fullprof program is an evidence for structural phase transition. The structural results suggest the presence of Co4+ and Co3+ ions that help to overall oxidation activity of these samples and this feature is higher for x=0.60. When substituting Mg and Ca in LaCoO3 the catalytic activity of CO oxidation shifts to lower temperature. La0.5Ca0.50Co0.5Mg0.5O3 nanocatalyst exhibits excellent CO conversion rate of 95% at 267 degrees C. These results suggest that the lower temperature conversion of the sample x=0.60 in series of La1-xCaxCo1-yMgyO3 can be mainly attributed to the presence of (i) higher Co3+-O-Co4+ couples, (ii) a structure having the higher tolerance factor, and (iii) a lower crystallite size.