The cyclic softening during high temperature Low Cycle Fatigue (LCF) test of a Ni-base superalloy Rene 80 was studied. The LCF tests were conducted with total-strain (0.6~0.9%) control mode at 760℃, 871℃ and 982℃ using high temperature extensometer. The electrolytic extraction was carried out to observe the microstructural change of γ`. The alloy showed different cyclic softening with temperature and strain range. The quantity of cyclic softening increased with the test temperature and decreased with the strain range. The extent of cyclic softening was analyzed by the theories of strengthening. The theoretical prediction based on the precipitate hardening alone underestimated whereas the one based on the coherency hardening overestimated the amount of cyclic softening. Therefore the actual cyclic softening mechanism was attributed to the combination of the two mechanisms.