AbstractThermal fatigue of the divertor plate is one of the key issues which governs the lifetime of the divertor plate. A thermal cycling experiment of divertor mock-ups was carried out to investigate the thermal fatigue behavior of the divertor structure in a high heat flux test facility at Japan Atomic Energy Research Institute (JAERI). A cyclic heat flux of 5 MW/m2 was loaded onto the mock-ups to simulate the steady state thermal condition of ITER divertor plate. A pulse duration of 30 s was selected so that the mock-ups reach thermal steady state. The mock-ups showed water leakage due to thermal fatigue cracking of the cooling tube after 400 cycles. The fatigue crack was observed at the top of the cooling tube. According to a numerical analysis, the maximum strain amplitude was over 5% at the top of the cooling tube. The cracking of the cooling tube was caused by the large strain concentration due to structural discontinuity of the copper heat sink. It was found that the heat sink block should be separated into smaller blocks to reduce the strain concentration at the cooling tube.