By using the phase field model for the solidification of multi-component alloys and coupling with real thermodynamic data, the dendritic morphology transition and the dendritic micro-segregation of Ni-Al-Nb ternary alloys are simulated in two cases, i.e., varying the alloy composition at a fixed under-cooling and varying the undercooling at a fixed alloy composition. The simulated results indicate that with the increase of the dimensionless undercooling U (U=ΔT/ΔT0, where ΔT is the undercooling and ΔT0 the temperature interval between the solidus and liquidus), the dendritic morphology transfers from dendritic to globular growth in both cases. As to the dendritic micro-segregation, both cases present a regularity of increasing at first and then decreasing. By using the phase field model for the solidification of multi-component alloys and coupling with real thermodynamic data, the dendritic morphology transition and the dendritic micro-segregation of Ni-Al-Nb ternary alloys are simulated in two cases, ie, varying the alloy composition at a fixed under-cooling and varying the under cooling at a fixed alloy composition. The simulated results indicate that with the increase of the dimensionless under cooling U (U = ΔT / ΔT0, where ΔT is the undercooling and ΔT0 the temperature interval between the solidus and the liquidus), the dendritic morphology transfers from dendritic to globular growth in both cases. As to the dendritic micro-segregation, both cases present a regularity of increasing at first and then decreasing.