The mechanical properties of quench-tempered high-strength low-alloy steels are commonly optimized by fine and dispersively distributed alloy carbides. The role of the alloying elements in determining the alloy carbide precipitation sequence is of great significance. The co-addition of carbide-forming elements such as Mo, V and Nb complicates the precipitation behavior. The mutual inter--solutions and growth rates of various MC- and/or M₂C-type carbides are qualitatively affected by the intrinsic solubility and diffusion at certain tempering condition. However, comprehensive study of the precipitation sequence must be followed with atomic scale resolution techniques. The 3D atom probe (3DAP) is a unique tool capable of obtaining chemical information at the atomic level, offering a powerful method to investigate microstructural and compositional changes occurring at nano-scale. And the sizes, morphology and composition of individual alloy carbide may be visualized and quantified by 3DAP. In this paper, a quenched Nb-V microalloyed steel was chosen to investigate the precipitation behavior of alloy carbide after tempering at 450-650 ℃ for different times. 3DAP, micro-hardness test and TEM were applied to characterize the phenomena of hardening and softening during tempering, and the composition evolution and growth behaviors of the alloyed carbides were also studied. The results indicated the second hardening of the 500-600 ℃ tempering martensite is mainly resulting from precipitation strengthening of alloy carbides. The alloy carbides composition dynamically changed with elevated temperature or prolonged time, that is, the stronger carbide-forming elements replaced or partly replaced the weaker ones. At first V and Nb replaced Mo, and then Nb partly replaced V, and at last the carbides with certain composition were formed. Tempering time has relatively less effect on the carbides composition compared with temperature. When the tempering temperature elevated or tempering time prolonged, the alloying elements can obtain adequate diffusion energy and/or time, the plate-like carbides preferentially grow along the radial direction, and then grow along the thickness direction and start to coarsen.