Tensile tests and strain-rate jump tests were carried out at several temperatures below room temperature on Cu processed by accumulative roll-bonding (ARB). The temperature dependences of the flow stress and the activation volume were determined. In contrast to conventional coarse-grained materials where the activation volume increases with increasing temperature, the ARB processed copper by 8 cycles with ultrafine-grains showed inverse temperature dependence of activation volume, i.e., decreased activation volume with increasing temperature. This inverse temperature dependence of the activation volume is discussed in terms of thermally activated dislocation bow-out from grain-boundaries.