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Experiments were conducted to investigate the development of an ultrafine grain structure during compression with oscillatory torsion processing of high-purity aluminum (99.9%) with an initial grain size of 75 μm. The samples were processed using different deformation parameters: torsion frequency ( f ) and compression rate ( v ). The samples were examined using a scanning electron microscope equipped with a field emission gun and an electron backscattered diffraction detector. The results suggest that for high-purity aluminum an ultrafine-grained microstructure was obtained after a total effective strain ( ɛ ft ) of 45 in samples deformed at f = 1.6 Hz and v = 0.04 mm/s. A quantitative study of the microstructural parameters showed that the area fraction of the ultrafine grains (<1 μm) ( A 1μm ) was 44%, the fraction of high-angle boundaries was 53%, and the average diameter of the grains was about 600 nm. The yield stress and ultimate tensile stress reached 127 and 137 MPa, respectively, after deformation at a total effective strain of 45. When the total effective strain reached 120, the mechanical strength of the material decreased. This suggests that the decrease in strength is associated with the operation of the recovery mechanism that decreases the boundary volume. [ABSTRACT FROM AUTHOR]