The present invention relates to a method of producing ethylene by separately collecting ethane and ethylene which are by-products of a propylene preparation process using propane dehydrogenation reaction and also combining an existing propylene preparation process with a process of converting ethane to ethylene. According to the present invention, valuable ethylene may be produced by converting most of ethane and ethylene, i.e., by-products of a propane dehydrogenation process, into ethylene without using the ethane as fuel, thereby improving process economic efficiency. Furthermore, a product line in the process of propane dehydrogenation reaction may be changed from a propylene product alone into two products, i.e., propylene and ethylene, and thus the operating conditions of a propylene dehydrogenation reactor and the operating conditions of a de-ethanizer may be adjusted according to the market situation, thereby increasing the production proportion of a favorable product line.