The formation process of ordered macropores in a prepatterned p-type silicon wafer was investigated. FE-SEM observation focusing on the early stage of the pore growth clarified the two-step process of macropore formation, whose first step was isotropic pore expansion from prepared etch pits and the second was anisotropic growth in the depth direction. Investigation using different etch-pit patterns revealed that ordered macropores with constant diameters wen, obtained when the spacing of etch pits was 3-8 mu m, while the spacing of 2 mu m resulted in the pore-wall collapse. The diameter and the wall thickness of macropores showed a tendency to decrease with decreasing the spacing of prepared etch pits, which could not be explained only from the width of the space-charge region. The resistance distribution in a silicon substrate was proposed as a parameter playing a crucial role in determination of the wall thickness. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.