The formation process of thin palladium on a porous alumina tube by MOCVD using palladium diacetate as the metal organic salt source was clarified and optimized. It was found that a palladium composite membrane with fewer defects could be prepared by keeping the sublimation temperature at 200 degrees C, the decomposition temperature at 220 degrees C, and the chamber pressure at 170 Pa during the MOCVD process. By CFD calculations, measurements of change in the electro-resistance and air leak tests along the palladium-deposited tube, it was shown that the plugging of pores in the support tube due to palladium deposition progressed from the base to the tip. The membrane prepared had a hydrogen permeability of 4.42 x 10(-9) mol . m(-1) . s(-1) . Pa-0.5 and a H-2/N-2 ideal separation factor of 11500 at 300 degrees C.