[en] The reactor thermal power of JOYO MK-III is to be increased from 100MWt to 140MWt due to high performance of reactor core. So, Dump Heat Exchanger(DHX) of MK-III was designed to improve its heat removal capability by changing U type heat transport tube arrays to Σ type tube arrays and increasing air flow. Natural frequency between support and support of MK-III DHX's tube arrays was about 15Hz, and Karman vortex shedding frequency of tube arrays was about 90Hz by Y.N.Chen's report. Then, a possibility of piling up of Karman vortex shedding frequency in high frequency mode was to be considered. And, air velocity of flow tube arrays is also increased compared to the MK-II DHX. Sodium leak accident of MONJU was caused by a flow-induced vibration of thermometer well. Therefore, the air flow test to tube arrays of MK-III DHX was conducted. High cycles fatigue damage of tube arrays was evaluated. Since, peak stress is below 2kg/mm², it can be said that high cycles fatigue damage of tube arrays by Karman vortex shedding vibration will not be caused. (J.P.N.)