NaBr and KBr single crystals were exposed to X-ray (W-target, 30 kV, 20 mA) at room temperature and strain rate cycling tests associated with ultrasonic oscillation were carried out for the crystals at 77-293 K. The stress decrement (Delta tau) due to oscillation and the stress change due to strain rate cycling have been measured during plastic deformation. The relative curve of a dagger tau and strain rate sensitivity (lambda) of flow stress has a stair-like shape for the two kinds of crystals. That is to say, the curve has two bending points and is divided into three regions: two plateau regions and the region between the two bending points, where lambda decreases gradually with increasing a dagger tau. The first region is a plateau within the small a dagger tau. This implies that X-ray-induced defects have weak interaction with dislocation and act as obstacles to dislocation motion. Furthermore, the dependence of stress decrement (tau (p)) at the first bending point on the activation volume (V) obtained from the difference between lambda in the first and second plateau regions reflects the interaction between dislocation and defects induced by the X-irradiation. The activation energy for the break-away of a dislocation from the defect could be obtained on the basis of tau (p)-V curve fitting the Barnett model to the experimental results. Then, the values of activation energy are 0.76 and 0.81 eV for NaBr and for KBr, respectively.