Al and Ti contained as impurities in amorphous Fe-Si-B alloy ribbons induce surface crystallization during casting and resultantly degrade the magnetic properties. This paper describes the effect of the oxygen concentration in casting atmosphere on the surface crystallization caused by Al and Ti. For alloys with Al, the surface crystallization is restrained by casting in a low oxygen atmosphere. The resultant magnetic properties are improved. In case of alloys with Ti, the low oxygen atmosphere fails to suppress the surface crystallization, and further degrades the magnetic properties. For Al, the crystallization appears to be initiated by oxides at the surface of the ribbon, while for Ti, the surface crystallization occurs regardless of the concentration of oxygen.