Palladium-based composite membranes have been used in hydrogen separation. In general, a dense Pd membrane was prepared on the porous substrate by an electroless plating process. Porous stainless steel (PSS) is suitable for the formation of composite membrane due to its thermal expansion coefficient close to the Pd-based films, ease to assemble with reactors, high mechanical strength, and thermal stability. However, it is difficult to prepare thin and defect-free membranes on PSS because of the wide pore-size distribution on its surface. As a concesequence, the thickness of the Pd membrane is largely dependent on the largest pores existed in PSS. To reduce the thickness of Pd membranes and maintain hydrogen permeability, aluminum oxide particles with particle size < 10 microm were employed to modify the pore size of PSS. Al₂O₃ particles with two different size distributions were used. The maximum pore size has decreased based on SEM observations after modification and helium flux through the porous substrates was kept. The thickness of the Pd membrane supported on 10 microm and 1 microm Al₂O₃- modified PSS are 23 microm and 19 microm, and the hydrogen flux at 773K and delta P=8 bar are 70.23 and 51.64 m³/m²h, respectively. For comparison, the thickness and hydrogen flux of the membrane without modification are 31.52 microm and 44.79 m³/m²h. The required thickness of a defect-free membrane was decreased significantly. Moreover, this fabrication can provide a higher hydrogen premeability through the membrane.