Abstract: The dissolution behavior of second phase particles in 960 MPa grade Nb-Ti microalloyed ultra-high strength steel after heat treatment at different temperatures for different times was studied by transmission electron microscope and energy dispersive X-ray analyzer. The results show that the test steel contained square TiN particles with size greater than 1 μm precipitated during solidification, square and ellipsoidal TiS or Ti (C, S) particles with size of 200 nm-1 μm, and square, spherical and ellipsoidal (Nb, Ti) (C, N) precipitates with size less than 500 nm precipitated by strain during forging. With the increase of heating temperature, the number of the second phase particles decreased and the size increased. With the extension of holding time, the number of the small second phase particles decreased, while that of large second phase particles increased and whose edges became blurred. These particles were composite precipitates containing niobium and titanium. Considering the full dissolution of carbonitride containing niobium and titanium and the suitable grain size of austenite, heating at 1 250℃ for 80 min was relatively appropriate for 960 MPa grade Nb-Ti microalloyed ultra-high strength steel.