Magnetic properties of (Sm0.5 Tb0.5) 2 Fe17-x Gax [mentioned subsequently as SmTbFe17-x Gax] have been investigated. All the compounds have formed in rhombohedral Th2 Zn17 structure with traces of α-Fe. The saturation magnetization decreases with increasing x and is attributed to the modification of the density of states of 3d band due to the low lying 3p band of Ga. The increase of Curie temperature up to x=3 and subsequent decrease is explained on the basis of band narrowing. The easy direction of magnetization is in the ab plane up to x=5 and changes along the c axis for x=7. © 2005 American Institute of Physics.