Designing method of an electronic device subjected to a chemical mechanical polishing process in a fabrication process thereof is conducted according to the steps of: dividing a substrate surface into first sub-regions; optimizing a coverage ratio of hard-to-polish regions in the first sub-regions to fall in a first predetermined range corresponding to the first sub-regions; dividing the substrate surface into second sub-regions different from the first sub-regions; and optimizing a coverage ratio of the hard-to-polish regions in the second sub-regions to fall in a second predetermined range corresponding to the second sub-regions, wherein patterns having a shorter edge of 5 .mu.m or less are excluded from the optimization.