In this paper, high purity cobalt (Co) powder and fine Y2O3 powder were used to fabricate Y2O3 doped Co bulks through ball milling and spark plasma sintering (SPS). The transformation from face-centered cubic (FCC) structure to hexagonal close-packed (HCP) structure during the cooling process after SPS was investigated. It has been shown that the addition of Y2O3 decreases significantly the grain size from 682 nm to 374 nm, since Y2O3 particles can effectively inhibit the grain growth by pinning the boundary migration. After doping the Y2O3, the FCC structured Co with a large volume fraction of over 70% can stably coexist with the HCP structured Co at room temperature, resulted from two possible reasons. First, Y2O3 particles hinder the dislocation movements. Second, Y2O3 particles occupy the deformation center of some defects in the FCC structure to reduce the number of potential nucleus of the HCP structure. In addition, due to the lower stacking fault energy of the FCC structured Co than that of the HCP structured Co, a lot of twins can be observed in the FCC grains in Y2O3 doped Co bulks after SPS. (C) 2017 Elsevier B.V. All rights reserved.