A method of forming a silicon nitride film which can form a silicon nitride film having a high film stress at a low process temperature is described herein. The method includes the steps of (a) supplying dichlorosilane into a reaction chamber containing a process object, thereby allowing chemical species originated from dichlorosilane as a precursor to be adsorbed on the process object; (b) hydrogenating chlorine contained in the chemical species, thereby removing the chlorine from the chemical species; and (c) supplying ammonia radicals into the reaction chamber, thereby nitriding the chemical species, from which the chlorine has been removed, by the ammonia radicals to, deposit resultant silicon nitride on the process object, wherein the steps (a), (b) and (c) are performed repeatedly for plural times in that order, thereby a silicon nitride film of a desired thickness is formed on a semiconductor wafer.