The Ni-Ta system was experimentally determined and thermodynamically optimized in the present work. In experimental work, 12 as-cast and 12 annealed alloys were measured using scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDX) and powder X-ray diffraction (XRD) techniques, and 5 crucial annealed alloys were determined using differential scanning calorimetry (DSC) method. It was found that Ni8Ta did not exist in both as-cast and annealed samples, and then two invariant reactions liq -> fcc(Ni) + Ni3Ta and liq -> Ni2Ta + mu-NiTa were modified. In the thermodynamic optimization, the solution phases liquid, fcc(Ni) and bcc(Ta) were treated with a substitutional solution model, three compounds Ni3Ta, Ni2Ta and NiTa2 were described using a two-sublattice model, and the compound mu-NiTa was modeled as (Ni,Ta)(1)Ta-4(Ni,Ta)(2)(Ni,Ta)(6) using a four-sublattice model. A set of self-consistent thermodynamic parameters of the Ni-Ta system was obtained.