Abstract: An investigation on the effect of Ce on the solidification structure of an Fe-Ni base Invar expansion alloy shows that great amount of high-melting compounds (Ce₂O₃) with a size of 2.0 μm form in the alloy after Ce addition. Based on the theory of lattice misfit, the lattice misfit of low-index surfaces between Ce₂O₃ and the alloy is 7.1%, which is relatively low; with Ce₂O₃ as heterogeneous nucleation, therefore, the solidification structure of the alloy changes from complete dendrite grains to complete equiaxed grains. There are Ce₂O₃, Ce₂O₂S and CeS in the interface of equiaxed grains, which prevent from the growth of crystal grains.