Low activation material is one of the important factors for constructing high power fusion reactors in future. Unexpected activation, however, may be produced through sequential reactions due to charged particles created by primary neutron reactions. In the present work, the effect of the sequential activation reaction was studied for candidate low activation materials of a fusion demo-reactor. The calculations were conducted by the ACT4 code developed in JAEA for the activation analysis of fusion reactor designs and revised for dealing with the sequential activation reactions. The results say that the real dose rate around vanadium alloy becomes larger after the cooling for 3 years by considering the reaction. Although metal hydrate is regarded as an excellent low activation shield material, the reactions due to recoil protons are influential and the dose rate around vanadium hydrate is several orders of magnitude larger than the value calculated without the sequential process after 2 weeks cooling. In case of liquid breeders, the effect of sequential reactions is popularly observed and it affects the breeder reprocessing and the shield design of circulation loop.