The microstructure of Mo was significantly refined by high pressure torsion to verify its irradiation tolerance in comparison with its micrograined counterpart. After deformation microhardness increased from 231 Hv0.2 for a microgarined sample to 542 and 558 Hv0.2, respectively after one and five rotations. Concurrently, the grain refinement was observed, as the grain size decreased with the increase of the deformation degree down to 480 and 110 nm, respectively for one and five rotations. Subsequently, deformed Mo and a micrograined one were irradiated by He ions to the dose of 8 × 10¹⁶/cm ² to verify their potential application as fusion mirrors. Irradiations were followed by reflectivity measurements in the 300–2400 nm range with a dual beam spectrometer. The measurements revealed that the applied dose causes a decrease in total reflectivity of the micrograined sample, whereas the total reflectivity of deformed samples decreases by additional 2.5%. Nanohardness measurements, detailed microscopy observations using focused ion beam and scanning transmission electron microscope as well as positron annihilation spectroscopy investigations were performed to elucidate changes in the microstructure and understand the different mechanisms of bubble creation after irradiation in micrograined and high pressure torsion processed samples.