Abstract: The structure and lattice strain of a series of [(La_1-ySr_y)_1-z□_z][Co_0.75Me_0.25]○_3-δ□_δ] ABO₃type compounds have been studied by polycrystalline X-ray diffrac tome try. In this structure, La³⁺,Sr²⁺and Ca 14% vacancaies occupy Asites randomly. Co and Me ions distributed at random in the Bsites and O^2-and its vacancies in the ○ sites. While the Sr²⁺content in the Asite increases, some of the Co ions change their valence from Ⅱ to Ⅲ and finally to Ⅳ, at this time both the lattice strain <ε₁₁₀²>^1/2 and the activity for COoxidation rapidly increases. Therefore the lattice stain <ε₁₁₀²>^1/2 could be regarded as a parameter to characterize the modifying effect of Asite substitution on the catalytic properties of these compounds.