Strain-rate and temperature dependences of deformation behavior of ultrafine-grained (UFGed) AZ61Mg alloy were examined. For this purpose, AZ61Mg alloy specimens were subjected to multi-directional forging (MDFing) under decreasing temperature conditions to have various grain sizes. The average grain sizes attained by MDFing to 1, 3, 6 and 9 passes were approximately 8, 2, 0.5 and 0.3 μ m, respectively. A superior balance of the mechanical properties of strength and ductility at room temperature was achieved by MDFing to three passes and over. The strain-rate sensitivity of mechanical properties of the MDFed specimens increased with decreasing grain size. The specimens with grain sizes of 0.5 and 0.3 μ m (0.5 or 0.3 specimen) exhibited stronger strain-rate and temperature dependences of total elongation than those with grain sizes of 8 and 2 μ m (8 or 2 specimen). This can be partially ascribed to grain-boundary sliding, since an AFM observation revealed the occurrence of room-temperature grain-boundary sliding in the 0.3 specimen. The activation volume V * for the 8, 2 and 0.5 specimen increased with increasing temperature, while the 0.3 specimen exhibited an inverse temperature dependence of V *. This suggests a change in deformation mechanism with decreasing grain size as well as the occurrence of grain-boundary sliding.