The crystal structures of Al2O3 and Y-AI-O particles in the as-hot pressed (as-hped) and as-heat treated oxide dispersion strengthened (ODS) steel were identified in this paper using high resolution transmission electron microscopy (HRTEM) techniques. The carbon extraction replicas were utilized to investigate the composition and crystal structure of the oxide particles in order to exclude the influence of the matrix. In the as-hped ODS steel, both of the gamma-A1203 and alpha-Al2O3 particles with different size were identified, while, only the large alpha-Al2O3 particles were found in the as-heat treated ODS steel. In addition, YAlO3 and Y3Al5O12 with body-centred cubic structure were the mainly observed Y-Al-O particles in the as-hped ODS steel, and large amounts of Y4Al2P9 particles with a monoclinic structure generated in the ODS steel during annealing. (C) 2017 Elsevier B.V. All rights reserved.