A SUS316 and a JPCA were irradiated at 643, 793 and 873K to 30 dpa in FFTF (Fast Flux Test Facility)/MOTA (Material Open Test Assembly) during its cycle 11 operation to investigate the effect of high helium generation rate on cavity formation under neutron irradiation. In FFTF/MOTA, (n, α) reaction of boron isotope can produce a large amount of helium atoms in SUS316 and JPCA. In all of temperature conditions, cavities were observed in both alloys with different contens of ^B. In view of number density, size distribution and mean size of cavity in the alloys, the effect of helium generation rate on the nuclation and the growth of cavity is discussed
