We report on allotropic phase transformation and nanograin refinement of Ge by severe plastic deformation using high-pressure torsion (HPT) under a pressure of 24 GPa. No appreciable formation of metastable phases occurred under compression prior to torsion, while a diamond cubic Ge-I phase and a tetragonal Ge-III phase were observed in the HPT-processed samples. The formation of the Ge-III phase was enhanced by introduction of shear strain. TEM observations revealed that HPT-processed samples consisted of micro- and nanograins. It was indicated that grain refinement occurred due to the introduction of high density of lattice defects in metallic Ge-II during HPT processing, and then Ge-II transformed not only back to Ge-I but also to metastable Ge-III upon unloading. The Ge-III phase reversely transformed to Ge-I by intense Ar-ion laser irradiation or by thermal annealing. No appreciable photoluminescence (PL) was observed from the HPT-processed sample, while a broad PL peak in the range of 600-800 nm appeared after intense laser irradiation. A similar PL peak was also observed from thermally annealed samples. These results suggest that the appearance of the PL peak arises from Ge-I nanograins.