The optical activity and birefringence associated with elastic and plastic deformation of alkali halide single crystals were studied by means of transmission of monochromatic light in the visible region between crossed Nichols. Below the elastic limit, the optical axis is the direction of stress application. For permanently deformed crystals, the optical axes are the [110] directions regardless of the directions of original stress application, and the associated birefringence is shown to be dependent on the modes of plastic deformation. The origin for optical activity in both elastically and plastically deformed materials is not yet clear and is probably associated with perturbation by local electric or magnetic fields of charged dislocations present in the material under investigation.