The influence of grain boundary (GB) modification on the plasticity and plasticizing mechanism of commercial pure nickel was studied in this paper. Thermo-mechanical process including compressing (with strain of 5%–30%) and annealing was carried out on cylindrical nickel samples to regulate the microstructure of GBs. Hardness measurements were conducted using Vickers hardness tester. The effects of different strains on the special boundary fraction, grain boundary character distribution (GBCD), the connectivity of the boundary network and Schmid factor were analyzed by electron backscatter diffraction (EBSD). And then, the relationship between GB characters and plasticity which was represented by Vickers hardness and Schmid factor was obtained. It was found from experiment results that the plasticity of nickel samples increased firstly and then decreased with the rising of strain from 5% to 25% after compressing and annealing. The optimal plasticity of nickel was achieved when the strain was 10%, in which the special boundaries fraction (73.48%) was twice that of the as-received sample (37.51%). In this case, the Vickers hardness reduced by 25% than that of the as-received one and the average Schmid factor reached the highest. The representation results of plasticity employing Vickers hardness obtained by physical method was consistent with the average Schmid factor acquired by microscopic EBSD technique. It was also found that the size of grain clusters and the length fraction of Σ3ic to Σ3c twin boundaries both played an important part in the plasticizing by grain boundary modification. The large cluster size and high fraction of Σ3ic to Σ3c boundaries led to the fine plasticity. The effect of grain boundary modification on plasticity and the plasticizing mechanism through thermo-mechanical processing in pure nickel were illustrated in this paper based on above microstructure characterization researches. It was of great significance to plasticize the pure nickel and to promote the application of grain boundary modification on plasticization in metal billets.