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Abstract: V–4Cr–4Ti, F82H, Ni and Cu were irradiated with fission and fusion neutrons at room temperature and 473K. Defect structures were analyzed and compared using positron annihilation lifetime measurement, and microstructural evolution was discussed. The mean lifetime of positrons (the total amount of residual defects) increased with the irradiation dose. The effect of cascade impact was detected in Ni at room temperature. The size and the number of vacancy clusters were not affected by the displacement rate in the fission neutron irradiation at 473K for the metals studied. The vacancy clusters were not formed in V–4Cr–4Ti irradiated at 473K in the range of 10−6−10−3 dpa. In F82H irradiated at 473K, the defect evolution was prevented by pre-existing defects. The mean lifetime of positrons in fission neutron irradiation was longer than that in fusion neutron irradiation in V–4Cr–4Ti at 473K. It was interpreted that more closely situated subcascades were formed in the fusion neutron irradiation and subcascades interacted with each other, and consequently the vacancy clusters did not grow larger. [Copyright &y& Elsevier]