The recent progress on our multiscale modeling to understand radiation damage processes in materials during irradiation is reviewed. The energies of He-V cluster formation in Fe were evaluated using a molecular dynamics (MD) simulation technique that employed interatomic potentials partially developed by first-principle (FP) calculations. Using the calculated energies, the longer timescale behavior of He-V clusters in Fe was investigated using a kinetic Monte-Carlo (KMC) simulation technique. The FP-MD-KMC scheme provided us significant information on the thermal stability of a He-V cluster in Fe as a function of the helium-to-vacancy ratio of the cluster.