Band-structure calculations based on the Korringa - Kohn - Rostoker coherent-potential approximation method are used to study the electronic structure and magnetic properties of FCC Ni - Ru alloy with a Ru concentration of 20 at.%. The results show that the ruthenium is ferromagnetic in Ni with a moment of about in the low-Ru-concentration region. The density of states calculated in the paramagnetic state shows that the strong d - d hybridization between Ni and Ru causes spin splitting of Ru, which is similar to the BCC Fe - Ru alloy. However, the local magnetic moment of Ru in the FCC Ni - Fu alloy drops abruptly as the Ru concentration increases. It is due to the non-magnetic character of FCC Ru, although BCC Ru is ferromagnetic. We clarified the reason why the average magnetic moment of the FCC Ni - Ru alloy drastically decreases on Ru addition.