Molecular dynamics (MD) simulation of displacement cascades in Fe-0.1%He are used to investigate the synergistic effects of primary knock-on atoms (PKA) and He on the irradiation damage production at temperatures between 10 K and 523 K. The results indicate that the vacancies produced by cascades are all in the central region of displacement cascade. The presence of He does not influence Frenkel pair production. During cascade evolution all displaced Fe and He atoms combine with each other to form Fe-He or Fe-Fe interstitials dumbbell as well as interstitial clusters. The number and size of interstitial clusters increase with the PKA energy and temperature. A few large clusters consisting of a large number of Fe interstitials with a few helium atoms are formed, the rest being Fe-He clusters at small and medium sizes, The interstitial dumbbell of Fe-Fe and Fe-He are in the < 100 > and < 110 > lattice direction, respectively. (c) 2007 Elsevier B.V. All rights reserved.