An apparatus for forming longitudinal thermal fatigue cracks. A heating unit has an induction coil disposed adjacent to an outer circumference of one side of a tubular test piece, on an inner surface of which a notch is formed. A cooling unit has a cooling water pump and a cooling water hose which forcibly injects cooling water from a cooling water storage source into an inner circumference of the tubular test piece. A control unit controls operation of the heating and cooling units. A cooling block partially encloses the outer circumference of the tubular test piece so as to control a magnitude of y-axial stress, is supplied with a cooling source of fluid or gas from an outside so as to repetitively cool the tubular test piece heated by the heating unit to adjust a temperature gradient, and has a longitudinal slit for controlling crack positions.