Abstract: A method of nitriding by high temperature electrolysis in which the works to be nitrided are dipped into a molten cyanide bath which contains low grade oxides of titanium and/or zirconium in a hot dispersed form as catalyst and maintained at a temperature range of 760.degree.-850.degree. C., and a direct current of 10-100 A/dm.sup.2 density is conducted through said cyanide bath, using said works as anodes. Thus a nitrided case of exceedingly deep and high hardness is obtained onto the work surfaces within a very short operating time, due to the rapid nitriding reaction at such high treating temperature. The deformation of the nitrided work is very small despite such a high treating temperature. Further, the method is applicable to those materials like titanium, high alloy steels, cast irons, etc., which have been considered difficult to be nitrided by the conventional method.