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Plastic deformation behaviour in Ti–54.7 at.%Al and Ti–58.0 at.%Al single crystals was examined around and above the anomalous strengthening peak temperature (Tp) focusing on the effect of Al5Ti3 superstructure. The Al5Ti3 superstructure developed in the L10 matrix of Ti–58.0 at.%Al, and the size of the Al5Ti3 phase once increased during annealing at 800°C and then decreased with increasing temperature, while no significant evidence of the Al5Ti3 particles was obtained in Ti–54.7 at.%Al from TEM observation although diffuse scattering corresponding to the spots for the Al5Ti3 superstructure was observed. The transition of slip plane for ½lang110] ordinary dislocations from {111} to {110) and/or (001) occurred at and above Tp due to anisotropy of anti-phase boundary energies on {111}, {110) and (001) in the Al5Ti3 superstructure. Anomalous strengthening is related to the development of this superstructure which may assist the cross-slip of some parts of ½lang110] ordinary dislocations onto {110) and/or (001) resulting in the formation of dragging points to the motion of the dislocations.