During irradiation, some of the mobile point defects induced by irradiation migrate to and pin down dislocations. Whereas a behavior of these pinning points on dislocations is subject to controversy at present,1, 2the number of these pinning points is considered to increase, at least, during irradiation. Normally, since the pinning reduces the motion of the dislocations, the elastic modulus of metals increases during irradiation,1as long as a dislocation string vibration is not in a region of over-damping (normal modulus change). On the contrary, if the dislocation loop length and/or the viscous drag constant of dislocation line are large enough (far in a region of over-damping), the pinning increases the dislocation motion in phase with the applied stress and thus decreases the elastic modulus of the specimen (anomalous modulus change). The above mentioned is true for the pinning where the dislocation at the pinning is either firmly fixed3or able to move with a large viscous drag4or with an elastic restoring force.5Such an anomalous modulus change was observed by True11 and Granato6for slightly deformed Al and NaCl single crystals in ultrasonic measurements, and expected to occur usually only in MHz frequency range. The purpose of this note is to report the observation of an anomalous modulus change in low frequency measurements (∼600 Hz) and to discuss the implication of this finding on the dislocation motion in low frequency.