The structure, magnetic properties and polar Kerr effects ofFe100−xSix(0⩽x⩽70)alloy films prepared by ion-beam co-sputtering were studied. We find that the saturation magnetization of these films decreases with increasingx,and a plateau of constant magnetization occurs as22⩽x⩽28.Their polar Kerr rotation&thgr;kdecreases with increasingxasx⩽16.2andx⩾30,but increases with increasingxas20⩽x⩽28.In the measuring wavelength range of 400–800 nm, the&thgr;kofFe72Si28film is 40&percent; to 50&percent; larger than that of pure Fe film. We think that amorphization may play an important role in enlarging the polar Kerr rotation of these films. ©1998 American Institute of Physics.