Provided is a method of inhibiting the formation of coke in an ethylene dichloride to vinyl chloride monomer pyrolysis cracker. The coke formation, which occurs during an ethylene dichloride pyrolysis reaction, is inhibited by contacting a heat-transfer surface of the cracker and a boron compound. The amount of coke generated when a coke formation-inhibiting material is used is 50% or less of that when a coke formation-inhibiting material is not used. In this case, however, the ethylene chloride conversion and the selectivity to a vinyl chloride monomer during the pyrolysis reaction are not affected. Accordingly, the efficiency of the pyrolysis cracker can be maximized.