Understanding　the　role　of　sintering　aids　during　microstructure　evolution　is　critical　to　the　manufacture　of　densified　SiC　fibers.　A　variety　of　TEM　characterization　techniques　are　combined　to　investigate　grain　growth　behavior　in　iron-doped　SiC　fibers.　Ultra-large　SiC　grains　in　micron　size,　as　the　self-assembly　of　nano　sub-grains　into　a　similar　orientation,　were　consistently　discovered　at　the　surface　and　indicative　of　abnormal　grain　growth.　The　growth　front　consisted　of　polycrystalline　nanograins　wetted　by　iron-rich　particles,　where　several　sub-grains　were　found　to　unify　their　(111)　planes　with　a　misorientation　angle　less　than　10　degrees,　indicating　grain　rotation　at　the　growth　front.　It　is　proposed　that　iron-rich　particles　form　a　quasi-liquid　interfacial　phase　during　sintering,　which　facilitates　coherent　attachment　of　grains　and　results　in　fast　grain　growth　using　neighboring　irregular-shaped　nanograins　as　building　blocks.　The　imperfect　ordered　coalescence　of　nanograins　introduces　structural　hetero-geneities,　including　low　angle　grain　boundaries　and　porosities.