Abstract: The invention refers to a tool and device for cold expansion of holes. The tool comprises a mandrel (1) split so, that at least three symmetrical segments (49) are formed in it. In an axial hole (8) of the mandrel a mobile pin (3) is positioned, and the working part of the mandrel is formed by two conical surfaces, connected by one round surface. The device comprises a cylinder (4) with a piston (5) and a piston rod (6), to the end of which the mandrel (1) is clamped, a second piston (10), inserted in a hole (8) in the piston (5) and the piston rod, connected to a second piston rod, the end of which is statically clamped to the pin. The tool and device provide one and the same tightness between the mandrel and the machined holes and reduce the number of technological operations.