Abstract: The precipitation behaviors of carbonitrides in a V-Ti-N microalloyed steel for nonquenched/tempered seamless oil-well tubes were calculated by the equilibrium principle of a three-phase miscibility gap including the austenite and two carbonitrides. The thermodynamic calculations indicate that the precipitates formed first at 1 473℃ are TiN particles, subsequently some of them formed at high temperatures maintain the chemical properties down to low temperatures, the others gradually transform to complex (Ti_xV_1-x) (C_yN_l-y) carbonitrides. In addition, the precipitates formed at 846℃ are V-and C-rich V(C_xN_1-x) carbonitrides. The formation sequence is verified by experimental data. These results are more consistent with the viewpoint that, the precipitation behavior of vanadium in the austenite matrix below 800℃ in medium-carbon V-bearing microalloyed steels plays a great role in controlling the decomposition of austenite.