Abstract: An apparatus for sizing of tubes under the pressure of fluid comprises a split die (2) with a lock mounted on a bed (1) and provided with a horizontally arranged sizing channel (3) corresponding to the shape of a tube (7), a mandrel (6) for setting the tube (7) in the sizing channel (3) coaxially therewith, a means for delivery of fluid into a clearance (10) between the mandrel (6) and the tube (7), packings (19) for sealing the clearance (10) between the mandrel (6) and the tube (7), a hydraulic striker unit (24) for building up the pressure of fluid in the clearance (10), incorporating a working chamber (34) filled with a fluid and a vertically arranged receiver (30) with a barrel (25) wherein slides a striking ram (26) which at the impact builds up the pressure of fluid in the working chamber (34) connected to the sizing channel (3) of the die ( 2) through the medium of an adapter (36) joined to the sizing channel (3) and provided with a piston (37) serving for isolating the fluid filling the working chamb