We report the microstructural evolution during irradiation of FePt and FePt 25at.% thin films sputter deposited onto electron transparent silicon monoxide substrates. The films were studied in situ for 500keV Kr+ irradiation up to a fluence of 1015ions/cm2 or 4displacements∕atom (dpa). Upon irradiation to approximately 1dpa, the initial disconnected granular morphology became continuous. In particular, for FePt, accelerated grain growth was observed once the continuous morphology was achieved during ambient temperature irradiation. No atomistic (chemical) ordering from the as-deposited A1 phase into either the L10 FePt or L12 Fe3Pt phases was observed during ambient temperature irradiation. After irradiation, the specimens were then in situ annealed. The intermetallic ordering temperature, compared to that of an unirradiated film, was lowered by ≈200°C for FePt 25at.%. No decrease in the ordering temperature was observed for irradiated FePt. The rate of FePt grain growth during annealing was very similar for both irradiated and unirradiated films over the 25–650°C temperature range investigated.