The tool shank contains a taper member, a flange member, a holder member, a clamped member, and a number of slits. The taper member has a conic shape extended along an axial direction with a top section and a bottom section. The flange member is beneath the bottom section of the taper member. The holder member is extended downward from a bottom side of the flange member along the axial direction. The clamped member is extended upward from the top section of the taper member. The slits are configured at intervals around the bottom section surrounding the axial direction. Each slit defines a slightly outwardly bulge. The bulges increase the contact area with and avoid gaps to a tapered socket of a spindle. As such, the tool shank is tightly jointed to the spindle during high-speed rotation without causing vibration and damage.