Abstract: Borided layers on the surface of Armco iron and eutectoid carbon Steel were eutectified in furnace and then scanned continuously with laser beam of high power densities. The laser melted metal were cooled and then solidified with extra-ordinary speeds by the heat extraction action of the cold substrale. It was found that the eutectic structures are composed of α-Fe+Fe₂B phases in the Fe-B alloy layer and of α-Fe + Fe₃(C, B) phases in the Fe-C-B layer. Fe₂B is in the square rod-shaped form and Fe₃(C, B) in the lamellar form respectively after furnace treatment. The phase constitution and the morphologies are not altered, but the inter-rod and interlamellar spacings are reduced by 40 and 100 times respectively after the laser melt quenching. The hardness of eutectic structures are thus greatly enhanced and the hardness-dimension relationship for each alloy system obeys the Hall-Petch equation.