Inconel 625® contained Ni 61%, Cr 21.5%, Mo 9% and so on is one of the nickel base super heat resistant alloys and is used for parts in food machine. The nickel base alloys are generally difficult-to-cut materials because of high thermal resistance, high work-hardening, high chemical activity and low thermal conductivity. In tapping, furthermore, the tool failure such as chippings, cracks and tool fractures has been frequently observed owing to low rigidity of tool. This paper deals with the influences of surface treatment, tool geometry etc. on the cutting phenomena such as the tool failure, the cutting force and so on when tapping Inconel 625® using the high speed steel tool. The damage of oxidizing tool is slower than that of nitriding tool. The large thrust force in backward tapping causes the rapid tool fractures. The smaller helix angle leads to the lower thrust force in backward tapping.