Structural transition during in-situ annealing of monoclinic-Yttria (m-Y2O3) is studied using X-Ray Diffraction (XRD) and Transmission Electron Microscopy (TEM). The m-Y2O3 has been prepared by high energy ball milling of cubic-Y2O3 (c-Y2O3). Monoclinic to cubic (m → c) phase transition with detailed crystallite size analysis from the Rietveld fit on XRD profiles and strain analysis from Geometric Phase Analysis (GPA) on High-Resolution phase contrast-TEM micrographs has been discussed. During annealing, the m- and c-phases of Y2O3 coexist up to ∼850 °C. The data has been compared with those reported earlier for the milling induced c → m transition. At 1000 °C, the phase is completely c-Y2O3. HRTEM time-lapse images have been employed to examine the growth mechanism involved as they develop into cubic crystallites. Grain rotation and defect-assisted grain alignment govern the sluggish growth of crystallites. Display omitted