Abstract: Three-dimen sional atom probe(3DAP) was employed to investigate carbonitride clusters in Nb-V micro-alloyed steels. There are great differences in compositions of these clusters with difference sizes. The results indicated that the carbon atoms congregated at the dislocation first,the vanadium and niobium atoms segregated to the place in turn,V-C and V-Nb-C clusters came into being successively,and finally (Nb,V)C complex phase was formed. The composition becomes asymmetry as the precipitates grow up. If nitrogen element exist,compound phase (Nb,V)(N,C) will be formed.