Methods and apparatus are provided for forming thin films for semiconductor devices, which enable supplying and removing reactants containing constituent elements of a thin film to be formed, by preheating and supplying a process gas and a purging gas at a predetermined temperature in forming the thin film on a substrate. For example, a method for forming a thin film includes supplying a first reactant to a chamber to chemically adsorb the first reactant onto a substrate, the first reactant being bubbled by a first gas that is preheated, purging the chamber to remove residues on the substrate having the first reactant chemically adsorbed, and forming the thin film by a means of chemical displacement by supplying a second reactant to the chamber to chemically adsorb the second reactant onto the substrate.