A study of the preparation and resonance properties of NiO doped yttrium iron garnet (NYIG) single crystals forms the subject of the present investigation. These crystals having the formula Y3NixGexFe5-2xO12, where X = 0.025 and 0.13 were prepared by the usual flux method. The resonance properties were measured on spherical samples at about 9100 Mc/s between -185 and +100°C. It is seen that at 20°C,K_{1}/Mdecreases with NiO and shows a tendency to become positive. TheK_{2}/Mis negative and seems to decrease with NiO. As the temperature is lowered,K_{1}/Mchanges sign and becomes increasingly positive, whereas,K_{2}/Mbecomes more negative. It is interesting to note that at low temperatures, 110 becomes the hard axis and 100 the easy one. The temperature dependence of ΔH is quite normal. The rise in thegfactor at a low temperature is very sharp. These properties are explained by supposing the presence of a small fraction of Ni²⁺ions in the tetrahedral (24d) sites where their ground orbital level is triply degenerate. This provokes a spin-orbit coupling which could contribute to the anisotropy ΔH and thegfactor. The presence of Ni²⁺ions in the 24d sites is further confirmed by the initial inciease in the lattice parameter with NiO addition. The possible ionic distributions are discussed.