The magnetic properties and Ferromagnetic resonance of the yttrium-lanthanum iron garnet system Y3(1-x)La3xFe5O12 were investigated at the compositions corresponding to x=0, 0.05, 0.10, 0.20, 0.30, 0.50, 0.75 and 1.00. The single-phase region with garnet structure determined by the X-ray powder pattern analysis and metallographic observations extends to about 0.10s, initial permeability μ0, coercive force Hc, effective g-factors geff and resonance line-width △H (at 3970 and 9160 Mc) were measured as a function of composition. The variation of σs with composition and the magnitude of μ0 as viewed from various magnetization mechanisms were discussed. From the frequency dependence of geff, the intrinsic g-factors and internal fields Hi of the materials were calculated. Exponential dependence of △H, Hi and Hc on x in certain composition region was observed experimentally and interpreted from the point of view of magnetic inhomogene-ities.