Effect of strain rate on grain refinement was studied in multidirectional forging (MDF) of a coarse-grained 7475 Al alloy at 490 degrees C under strain rates of 3 x 10(-4) s(-1) and 3 x 10(-2) s(-1). At a strain rate of 3 x 10(-4) s(-1), the stress - strain (sigma-epsilon) behavior shows significant work softening just after yielding and a steady-state flow at higher strains. The structural changes are characterized by development of deformation bands at early stages of deformation, followed by formation of a fine-grain structure in high strain in the whole material. The volume fraction of new grains increases with strain and approaches a value of about 0.85 over a strain of 3. At a higher strain rate of 3 x 10(-2) s(-1), in contrasts a steady-state flow following small flow softening appears at a relatively low strain. New grains are formed during steady state flow along original grain boundaries and the volume fraction reaches below 0.2 even at a strain of 6.3. The occurrence conditions and the mechanisms of grain refinement are discussed in detail.