The grain refinement process and phase transition phenomena of a cold-rolled zirconium have been systematically investigated. The grain refinement was accomplished via a sequential series of dislocation behaviors, reaching a final grain size of <100nm. The refinement process can be described as: (1) dislocation nucleation, multiplication and tangling inside the original coarse-grains, (2) formation of microbands via dislocation rearrangement, (3) further division of microbands into thin laths, (4) split of microbands and thin laths into subgrains and then nanograins. Two different types of HCP-FCC phase transitions were also discovered during the cold rolling process, with the transition mechanisms and their influences on the mechanical properties being discussed.