In this paper numerical and experimental studies of deformation and fracture of thin beryllium disks with a thickness of 5-100 μm are performed. Disks, intended for use as X-ray windows, were obtained by the method of severe cold deformation. It is shown that beryllium foils under loading by external pressure (or bulge test) have higher values of ultimate strains than in tensile tests. It is established that the fracture at bulge test occurs at values of the accumulated plastic strain amounting to 15-20%, what very high for beryllium.