High-quality ultra-fine grained (ufg) and nanocrystalline (nc) bulk Cu samples of proper sizes reliable for mechanical testing, with grain sizes (d) ranging from 720 down to 22 nm were prepared by means of room temperature ball-milling and consolidation processes. The specimens were subjected to compressive loading at the quasi-static strain rate of 10(-4) s(-1) to large strains (epsilon = 50%). The specimens prepared from the 10-h-milled powder (d = 32 nm) were tested at a wide range of strain rates (10(-4) to 1,860 s(-1)), and the strain rate sensitivity (SRS) of the material was determined as a function of strain. The strength and work-hardening behavior were dramatically influenced by change in the grain size; the strength approached similar to 900 MPa for the 30-h-milled Cu (d = 22 nm) at the strain level of similar to 50%. The SRS increased several fold as the grain size was reduced to 32 nm. Further, the results obtained in this study were compared with those of other investigators on ufg and nc Cu, to gain insights into the effect of different processing routes on the investigated material properties.