The objectives of this investigation were to study the mechanism of blistering in molybdenum alloys and to develop new molybdenum alloys for the first wall of fusion reactors. The materials used were molybdenum-carbon, molybdenum-rhenium and molybdenumniobium alloys. These materials were irradiated with argon or helium ions by a 400 kV Cockcroft type accelerator. The blistering characteristics were closely related to mechanical properties through an inter-bubble fracture mechanism by gas pressure. The temperature dependences of the morphology of blistering and flaking were observed by SEM and TEM. The mechanism for blistering is suggested from the observation of internal structures, and the important roll of dislocation cells together with bubbles in inter-bubble fracture to form blisters is discussed.