Provided is a method of forming a thin film having a target thickness T on a substrate by an atomic layer deposition (ALD) method. The method includes n processing conditions each having a film growth rate that is different from the others, and determining a1 to an that are cycles of a first processing condition to an n-th processing condition so that a value of |T−(a1×G1+a2×G2+ . . . +an×Gn)| is less than a minimum value among G1, G2, . . . , and Gn, where n is 2 or greater integer, G1, . . . , and Gn respectively denote a first film growth rate that is a film growth rate of the first processing condition, . . . and an n-th film growth rate that is a film growth rate of the n-th processing condition, and the film growth rate denotes a thickness of a film formed per a unit cycle in each of the processing conditions. The film forming method may precisely and uniformly control a thickness of the thin film when an ALD is performed.