In a method of forming active patterns, first patterns are formed in a first direction on a cell region of a substrate, and a second pattern is formed on a peripheral circuit region of the substrate. The first pattern extends in a third direction crossing the first direction. First masks are formed in the first direction on the first patterns, and a second mask is formed on the second pattern. The first mask extends in a fourth direction crossing the third direction. Third masks are formed between the first masks extending in the fourth direction. The first and second patterns are etched using the first to third masks to form third and fourth patterns. Upper portions of the substrate are etched using the third and fourth patterns to form first and second active patterns in the cell and peripheral circuit regions.