A downhole rotary cutting tool used with a rotational motor sub and powered by fluid pressure consisting of an elongate body housing having an axial piston cavity and plural, equi-spaced cutter blades mounted in plural equi-spaced longitudinal slots which communicate with the axial piston cavity. A piston in the piston cavity is subject to moving upward with application of fluid pressure to urge the cutter blades outboard into contact with surrounding tubular goods, and further increase of fluid pressure effects rotation of the cutting tool at relatively high speeds to cut the tubular goods.