In the present investigation, the effect of high-pressure torsion (HPT) deformation on the microstructures and mechanical properties of beta-Ti alloys containing isothermal omega phase was studied. X-ray diffractometry results indicated the intensity of omega peaks was the highest after the application of compressive stress. The subsequent torsional straining led to a pronounced decrease in the omega peak intensity. Optical microscope observations of the cross-section on the HPT discs revealed that numerous shear bands forms by HPT deformation. The density and thickness of the shear bands increased with increasing strain. White etching layer (WEL) appeared after 1 turn of HPT deformation and the WEL thickness increased with number of rotation. Microhardness measurements revealed that the hardness is lower inside the WEL (about 400 H-v) than the outside (about 460 H-v). The results suggest the decomposition of omega phase to beta phase.