This paper deals with the microstructure and properties of Nb-V-Ti micro-alloyed steels with different Nb,V and Ti contents by means of SEM and TEM.It mainly investigates the morphology and formation mechanism of the diphase precipitate and explores the effect of diphase precipitate on the microstructure and properties of the steels.The results show that the simultaneous addition of Nb and V with a small dosage may remarkably improve the ductility and toughness of the steel with 0. 15% Ti; and that,when the tensile strength is 1200MPa,the percentage elongation after fracture achieves 13% and the impact energy achieves 102 J at － 20℃.Two types of Nb- V-Ti particles in the diphase precipitate are revealed.One is the large particle with a grain size of about 100nm and the other is the ( Nb,V,Ti) C particle with a grain size of about 35 ～ 50nm.It is found that the latter is of stronger precipitation strengthening ability.