One of the little-studied problems of modern mechanics and the physics of fracture is the branching of the crack, which is observed in materials of a different nature. To experimentally determine the rate of crack branching velocity in steels, a series of tests were carried out on the fracture by internal pressure of steel thin-walled cylindrical shells (vessels) with measuring of the crack velocity by the method of breaking the conductive strips. A measurement setup was created on the basis of a precision converter “Tercon” signals of resistance thermometers and thermocouples, connected to a computer. Velocity measurements showed that in the case of straight-line propagation, the crack velocity is 350-541 m/s, and when crack branching, the crack velocity is 679-746 m/s. The results show that the crack branching velocity is a critical (limiting) crack propagation velocity, and the energy entering the top of a moving crack is not spent on increasing the crack velocity, but on creating new cracks by branching.