Abstract: The relations between the magnetostrictive properties and microstructure, iron content.x of the alloy Tb_0.27Dy_0.73Fe_x(x=1.65-2.0) in as-cast have been studied. The matrix phase of this alloy has MgCu₂ type structure. When x=1.90, the alloy in as-cast with small amount of second phase possesses a maximum values of λ₁₁,λ₁₁-λ and (dλ/dH)_m. When x is equal to or large than 1.95, the RFe₃ phase with Widmantanten-type morphology apppears, which leads to the decrease of magnetostrictive properties of the alloys. When iron content x is less than or equal to 1.85, the divorce eutectic structure or the two eutectic structure is observed, which causes the drop of the magnetostrictive properties of the alloys.