Abstract. Fe-6.5 %Si alloy has excellent soft magnetic properties but is hardly able to being produced by conventional hot-cold rolling processes. It is well known that a small amount of Al adding into Fe-Si alloys is benefit to enhancing the alloy's soft magnetic properties and its working performance. In the present work pure Al layers were deposited onto Fe-3 %Si thin sheets using direct current (DC) magnetron sputtering and followed by diffusion annealing to promote Al penetrating into low-Si steel substrates. Based on the experimental and simulation results, the mechanism of Al penetrated into low-Si steel substrate was subjected to rate-controlling and Si uphill diffusion only occurred under the influence of penetrated Al being gradient distributed. As deposited Al layer's thickness reached to 32 μm onto 0.23 mm Fe-3 %Si substrate and followed by a diffusion annealing at 1200 °C for 6 h, Fe-6.5 %(Si+Al) alloy can be achieved. Electron dispersive spectroscopy (EDS) analyses showed that both Si and penetrated Al profiles were evenly distributed along the depth of the prepared Fe-6.5 %(Si+Al) alloy, and the microstructure was also uniform and density. The resistivity of Fe-6.5 % (Si+Al) alloy was close to that of Fe-6.5 %Si alloy, but the working performance of the former was observed evidently better than the latter. Introduction Fe-6.5 % (weight percentage, hereinafter) Si alloy has excellent soft magnetic properties such as higher permeability, higher saturation magnetization, near to zero magnetostriction and lower core loss than conventional Fe-3 %Si steel sheets. However, due to the appearance of ordering brittle phases such as B2 or DO3, the high-Si alloys are hardly able to being produced by conventional hot-cold rolling processes [1][2][3][4][5]. Several techniques were developed to overcome the drawbacks, in an attempt to produce Fe-6.5 %Si thin sheets [6-9], but only chemical vapor deposition (CVD) realized low-scale commercial production, which is still far away from meeting the great demands required by electromagnetic applications [4]. Many researchers found that silicon steel sheets with a small amount of Al could optimize its magnetic properties and working performance. A. H. Kasama et al prepared Fe-3 %Si-3.5 %Al alloy by spray-formed approach, and found its magnetic properties were very well when compared with binary Fe-6.5 %Si. Claudemiro Bolfarini studied the magnetic properties of spray-formed Fe-6.5 %Si-1.0 %Al alloy and found that 1 % of Al added to the Fe-6.5 %Si alloy could suppress the generation of B2 structure and improve the Fe-Si alloy ductility [10]. J. Barros, et al assessed the feasibility of increasing the concentration of (Si, Al) in low-Si steel sheet by hot dipping and diffusion treatment [11]. All the above researches concluded that Al alloying into Fe-Si binary systems being advantage to its properties, but whether Fe-Si-Al alloys, especially Fe-6.5 % (Si+Al) alloy could be prepared by penetrated Al into Fe-3 %Si thin sheet was less reported, and what's the interaction e...