Coarsening induced reverse phase transformation of hafnia in polymer-derived Si-Hf-C-N-O ceramics was studied experimentally. X-ray diffraction and high resolution transmission electron microscopy were used to obtain information regarding nanocrystalline structure. Hafnia crystallites precipitated out from the amorphous ceramic matrix on heat treatment were observed to remain as tetragonal hafnia below a critical crystallite radius of 4 nm. Prolonged heat treatment has resulted in coarsening, as explained by Lifshitz-Slyozov-Wagner model, resulting in reverse phase transformation from tetragonal to monoclinic hafnia.