It has been confirmed that the fcc MC-type carbides such as VC and NbC have plate-like morphology and are mutually soluble when precipitated in tempering steel with martensite microstructure. Over-tempering makes the plate--like carbide change to spherical shape because of Ostwald coarsening. As coarsening is strongly linked to the diffusion rate of the carbide-forming elements, it is easy to understand that inhomogeneous structure may be formed when more kinds of elements were added, such as V, Nb and Mo. Besides, non-carbide-forming elements such as Si and Al tend to diffuse toward matrix. The morphology and lattice structure of carbide change simultaneously companying with the compositional redistribution of alloy elements. The detail compositional and nano-structural informations of the carbide can be obtained by 3DAP and HRTEM. In this paper, 3DAP and HRTEM were applied to characterize the composition, morphology and nanostructure of the carbide precipitated during 650 ℃ tempering of as-quenched Nb-V microalloyed steel. The results indicated that the martensite lath morphology was replaced by defect--free polygonal ferrite due to the recovery and recrystallization of the as-quenched microstructure. Simultaneously, the carbide-forming elements Mo and V diffused from smaller carbides to larger ones, resulting in the co-existence of carbides with different sizes and compositions. With the diffusion and redistribution of alloy elements, the prior-formed plate-like carbides grew along the radial direction, and a kind of transition carbide (P_outer) which is semi-coherent with ferrite matrix (M_bcc) was consequently formed. 3DAP constructional atom map demonstrated that Si and Al are rejected from the alloy carbide, whereas Mn and V were inhomogeneously distributed. That is, the coarsening carbide has a core-shell complex nanostructure, the core contains V, Mn, Mo and Nb, and the external shell contains Mo and Nb.