The Tb0.3Dy0.7Fe1.95-xNbx (x=0, 0.03, 0.06, 0.09) alloys were prepared by a high-vacuum arc melting furnace. The crystal structure, microstructure and magnetostriction of the alloys were studied. Results demonstrate that after Nb addition the matrix phase of the Tb0.3Dy0.7Fe1.95-xNbx (x=0.03, 0.06, 0.09) alloys consists predominantly of the cubic Laves phase with MgCu2 (C15-type) structure, and the lattice parameter of the Laves phase in the alloys is least affected. Nb is soluble negligibly in the matrix RFe2 and Re-rich phases, but dissolved in the RFe3 phase and formed into Re(Nb,Fe)3 phase when x=0.03. The formation of the NbFe2 phase as the primary phase make the solidifying liquid become rich in rare earths and suppress the formation of the deleterious RFe3 phase. The hexagonal C14-type Laves phase NbFe2 does not dissolve into the matrix RFe2 (C15-type) phase. The concentration of the element Nb affects the magnetostriction significantly. The improvement in magnetostriction is maximum for the element-added alloy with a low concentration of the element Nb (x=0.03) as compared to the parent alloy Tb0.3Dy0.7Fe1.95. However, the decrease in magnetostriction occurs with the increasing of Nb content which is due to the formation of paramagnetic phases NbFe2 and Re-rich phases. Whereas the magnetostriction has a little improvement as compared to the parent alloy Tb0.3Dy0.7Fe1.95.