After the Gd3(Fe1-xCox)29-yCry compound was formed, Sm3(Fe1-xCox)29 -yCry compounds were also successfully fabricated. Phase stability of R3(Fe1-xCox)29-yCr y(R=Gd,Sm) compounds was invest igated by x-ray diffraction and magnetic measurements. Atomic radius theory was employed to interpret why the 3∶29 compounds with high Co component had to cont ain more stabilizing elements. Less stabilizing elements are needed for those wi th larger atomic radius. However,a large atom of a stabilizing element augments the lattice parameter of the compound, which is disadvantageous to the stability of the 3∶29 phase. By studying the phase stability of R3(Fe1-x Cox)29-yCry(R=Gd,Sm) compounds, we have fabricated the Gd3(Fe1-xCox )29-yCry(0.4≤x≤1.0; 4.0≤y≤6.5) and Sm3(Fe1-xCox)29-y Cry(0.4≤x≤1.0; 4.5≤y≤7.5)compounds with room-temperature uniaxial magnetocrystalline anisotropy.