Plastic deformation behavior was investigated in near fully dense nanostructured and ultrafine-grained bulk samples of carbon steel (0.55 wt% C) under compression and tension tests. The specimens were obtained by hot pressure from mechanically milled powder at 400 and 500 0C. Subsequent heat treatments at temperatures going from 600 to 900 0C produced samples with ferrite grain sizes from 30 nm to 17 lm. Nanocrystalline grained steel samples presented very high strength with low ductility. Once, in the ultrafine range, as the ferritic grain size was increased, the strength was decreased and the ductility was improved. The porosity and carbon atoms within the structure were analyzed in order to explain the results of strength and strain obtained.