The crystal structure and magnetic properties of new K2NiF4-type oxides, La2-2xSr2xMg1-xRuxO4(1/3 less than or equal to x less than or equal to 1), have been investigated. Powder X-ray diffraction data were best fitted by the space group I4/mmm, which indicates that the arrangement of Mg2+ and Ru4+ is disordered. An increase in x from 1/3 first decreased the effective magnetic moment of ruthenium from 3.6 to 3.4 mu(B) then increased it up to 5.0 mu(B). The tetragonal distortion of(Mg,Ru)O-6 octahedra continuously decreased from 1.11 for x = 1/3 to 1.05 for x = 1.