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• The structural transformation of Y 2 O 3 during high-energy ball milling is investigated. • Complete phase transformation from cubic to monoclinic is achieved after 30 h of milling. • XRD and Rietveld refinement analysis for the quantification of phase fraction, crystallite size and strain. • Transformation mechanism confirmed by HRTEM and EELS analysis. • Transformation sequence is: Yc → Yc + Ym → Ym. • Monoclinic Y 2 O 3 is found to be much finer than its cubic allotrope. [Display omitted] In-depth understanding of the structural transformation mechanism in nanocrystalline yttria (Y 2 O 3) is crucial to design advanced materials satisfying a wide range of technical demands. Present study investigates the structural modification in cubic Y 2 O 3 nanopowders induced by high energy ball-milling as a function of milling duration. Y 2 O 3 nanopowder with an initial crystallite size of 25–50 nm was subjected to high energy ball milling (1000 rpm) for incremental time periods up to 30 h. X-ray diffraction (XRD) and Rietveld analysis were employed to study the phase evolution, crystallite size, and strain at every stages of milling. High Resolution Transmission Electron Microscopy (HR TEM) and Electron Energy Loss Spectroscopy (EELS) is utilized to realize the mechanisms involved in phase transformation. It is observed that as milling duration progresses, the cubic Y 2 O 3 gets refined and gradually transforms to the monoclinic phase. It is also apparent that at finer sizes, the monoclinic phase is preferred over the cubic one. The critical crystallite size for the monoclinic phase to stabilize is observed to be ~ 13 nm. Results show the complete structural modification after 30 h of milling duration. [ABSTRACT FROM AUTHOR]