Part of the phase boundaries of the solid phases in Ni-Ta system at 122 3K, 1173K and 1123K were determined by the diffusion couple and EMPA methods. Based on the experiment data and the information from literatures, the phase equilibrium relationship and thermodynamic properties of the system were analyzed and a complete thermodynamic description were obtained. Apart from the intermetallic phase Ni8Ta, All the inter-phases were treated with small homogeneity and the sub-lattice model was used to describe them. The calculated results reproduce the experiment information very well.