Microstructures of nickel deformed by the HPT up to strains of 12.0 have been examined by mean of transmission electron microscopy. Observations have been carried out in the longitudinal section for a proper demonstration of the structure morphology. A cell block structure delineated by extended planar boundaries has been clearly revealed over the strain range investigated. At low to medium strains (von Mises strain epsilon(vM)<1.0), the extended boundaries are often formed in two directions with respect to the shear direction; one is almost parallel to the shear direction, and the other is inclined at large angles to the shear direction. At strains epsilon(vM)>2.5, the extended boundaries are almost parallel to the torsion plane, forming a typical lamellar structure. The evolution of the microstructural parameters, boundary spacing and misorientation, is similar to that observed during the conventional torsion or rolling deformation. That is, the spacing decreases and the misorientation increases with increasing strain. An average spacing of 76nm. has been measured at the highest strain epsilon(vM)=12.0, and no saturation in the spacing is seen. Two new structural features, deformation twins and equiaxed grains have also been observed to contribute to the structural refinement at high strains.