Fe-6.5%Si (mass fraction) alloy is an important soft magnetic material due to its excellent magnetic properties. However, the existence of ordered structure in a great amount is the fundamental cause of poor ductility of the alloy, which restricts the application of the alloy seriously. To modify the microstructure and crystal structure of Fe-6.5%Si alloy by rare earth micro-alloying is one of the significant methods to reduce brittleness and improve plastic deformation ability of the alloy. Whereas, there still lack of elaborate studies on order degree reduction mechanism, ductility improvement evaluation and its connections to a varying microstructure, rare earth distribution, etc., caused by rare earth doping, which restricts a deep understanding on rare earth micro-alloying mechanism and its application in this alloy. In this work, influences of Ce content (mass fraction) on microstructure, ordered structures and warm tensile property of the as-cast alloy were investigated, and the ductility improvement mechanism of the alloy caused by Ce micro-alloying was analyzed. The results indicate that, there is no evident variation of as-cast microstructure when Ce content is below 150×10^-6, while the obvious microstructure refinement is observed when Ce content exceeds 210×10^-6. Ce addition reduces the alloy's order degree significantly and thus improves its warm tensile ductility obviously. Compared with Ce undoped specimens, average tensile elongation to failure at 400 ℃ increases from 7.4% to 10.1%, 19.3% and 23.0% by 62×10^-6, 150×10^-6 and 210×10^-6 Ce doping, respectively. Inter-granular brittle fracture characteristic occurs in fractured tensile specimens due to the obvious Ce enrichment at grain boundary when Ce content increases to 260×10^-6 and 790×10^-6, hence the average tensile elongation to failure at 400 ℃ reduces to 15.5% and 14.2%. A reasonable Ce content is within the range of (150~210)×10^-6 to improve effectively the ductility of Fe-6.5%Si alloy.