The procedures of X-ray diffractometry and peak profile analysis, together with differential scanning calorimetry (DSC), were used to measure the grain size, dislocation density and corresponding elastic energy stored in ultrafine-grained (UFG) nickel processed by different methods: equal channel angular pressing, high pressure torsion and their combinations. It has been shown that grain growth in these samples begins at temperatures in the range from 500 to 700 K and the measured activation energy for grain growth is close to the activation energy for grain boundary self-diffusion. A difference between the released enthalpy in DSC experiments and the elastic energy evaluated by x-ray diffractometry is attributed to the decrease in surface energy during grain growth in UFG nickel. The grain boundary (GB) surface energy of high angle boundaries was also evaluated.