The Ni-W binary system was assessed using critically evaluated experimental data with assistance from first principles analysis and the CALPHAD method. The solution phases (liquid, fcc-A1 and bcc-A2) were modeled using the substitutional regular solution model. The recently discovered Ni8W metastable phase was evaluated as Fe16C2-like martensite with three sublattices, and shown to be possibly stable according to first principles calculations. Ni8W was also modeled as an interstitial compound, but the model is not good because the solubility of tungsten in nickel is very low, especially at low temperatures. There is no experimental evidence for such low solubility. The other binary compounds Ni4W and Ni3W were assessed as stoichiometric ones. Compared independent experimental and first principles data agree well with the calculated phase diagram using updated thermodynamic parameters.