Abstract: The microstructures of 28CrMo quenched and tempered steel with different Mo mass fractions for oil country tubular goods (OCTG) were investigated. The type, content and chemical formula of precipitates with different Mo mass fractions were detected by physical chemistry phase analysis technique. Mass distribution within different particle size intervals was analyzed by small angle X-ray scattering. Carbides dissolution, precipitation law and distribution of alloying elements in matrix and various phases were considered. The relationship between them and properties was discussed. And thus a theoretical basis was provided to establish a reasonable process system. The results showed that carbides types were M3C and MC, and more than 80% particle size was 10-140 nm, the difference of anti-tempered performance was determined by Mo3C and VC in carbides phase.