In a thinner composition and a method of forming a photosensitive film, the thinner composition includes about 50 to about 90% by weight of propylene glycol monomethyl ether acetate, about 1 to about 20% by weight of propylene glycol monomethyl ether, about 1 to about 10% by weight of γ-butyrolactone, and about 1 to about 20% by weight of n-butyl acetate. The thinner composition may have a proper volatility and an improved ability to dissolve various types of photosensitive materials, and thus the thinner composition may be usefully employed in an edge bead rinse process, a rework process or a pre-wetting process.