The yttrium-ytterbium (Y-Yb) garnet system Y3-xYbxFe3O12 was investigated at the compositions for which x=0, 0.75, 1.50, 2.25 and 3.00. The samples were prepared by the co-precipitation method and sintered 7 hours in oxygen atmosphere at 1350℃ or 1400℃. Lattice constants and theoretical densities were determined from X-ray diffraction data. It was found that the whole system consisted of a single phase with the garnet structure, and the lattice constant decreased linearly from 12.371? (Y3Fe5O12) to 12.297? (Yb3Fe5O12). The static hysteresis loop, the initial permeability at 500 Kc, the saturation magnetization between liquid-air temperature to the Curie point and the magnetic spectrum between 500 Mc and 3000 Mc were observed as a function of compositions. It was found that all the samples had high bulk densities ( > 90 % theoretical densities) and high resistivities (109—1010ohm-cm), and that the saturation magnetization Vs temperature curves had positive slopes at 10W temperature for x ≥ 1.50.