Abstract: The yield strength of the vanadium micro-alloyed casing has rather high sensibility to the tempering tem- perature. Studied here in the essay is the influence by the carbonitride precipitation behavior on the yield strength of the vanadium micro-alloyed casing. The relevance between the effect of the vanadium micro-alloying and the Nb content is analyzed，and the VN and VC precipitation behaviors，and the strengthening effect of the precipitated phase are studied. It is regarded via the analysis that there exists an approximate quantitative relationship between the V content and the strength fluctuation of the materials as heat treated，i.e.，each time when the tempering temperature changes with 10 ℃，a 3~4 MPa additional yield strength fluctuation occurs as a result of 0.03% increasment in V content. Accordingly in actual production，the stability of the casing strength is expected to be improved by means of optimizing the chemical composi- tion design of the vanadium micro-alloyed steel types and properly controlling the product properties.