Provided are a composition for forming an organic light-emitting device, a method of preparing an organic light-emitting device, and an organic light-emitting device. The method of preparing an organic light-emitting device includes: forming an organic layer including an emission layer on a first electrode, wherein the forming of the organic layer includes performing a solution process using a composition for forming an organic light-emitting device, the composition for forming an organic light-emitting device includes n kinds of solvent, a solvent (Sv1) having the highest boiling point among the n kinds of solvent and a solvent (Sv2) having the second highest boiling point among the n kinds of solvent satisfy Equation 1, n is an integer from 2 to 10, and the solvent (Sv1) having the highest boiling point among the n kinds of solvent is a compound represented by Formula 1.