L12-ordered Cu3Au deformed by high pressure torsion (HPT) was investigated using Vickers microhardness and transmission electron microscopy (TEM). It is shown that the HPT deformation procedure can produce banded zones of nanograins with a grain size below 500 nm embedded within grains of several micrometers. Electron diffraction patterns and dark-field images using superlattice reflections reveal that the long-range order is destroyed completely at large strains (shear strain ∼ 50) whereas at lower strains (shear strain < 20) disordering occurs very locally on a nanometer scale. It is proposed that deformation-induced disordering is important for the formation of the fine-grained structure and occurs by antiphase boundary faults forming tubes. The observed increase of the microhardness with increasing shear strain can be correlated with a decrease of the long-range order and a decrease of the grain size.