The　understanding　of　the　microstructures　is　critical　to　the　design　of　SiC　fibers　with　excellent　thermal　stability　and　mechanical　properties.　We　introduce　a　density-sensitive　electron　microscopy　method,　high-angle　annular　dark　field　(HAADF)　imaging　to　study　the　microstructure　of　SiC　fibers　processed　at　a　variety　of　temperatures　ranging　from　1200　to　1800　degrees　C.　It　is　revealed　that,　irrespective　of　their　processing　temperatures,　SiC　grains　form　the　skeleton　of　SiC　fibers　that　are　surrounded　by　a　multiple　of　zones　with　low　HAADF　contrast　(low-density　zones,　LDZs).　LDZs　mainly　consist　of　amorphous　SiOC　phases　and　turbostratic　graphite.　As　a　result,　three　important　interfaces　that　dictate　grain　growth,　namely,　the　SiC/amorphous　SiOC　interface,　the　SiC/turbostratic　graphite　phase　boundary　and　the　SiC　high　angle　grain　boundary　(HAGBs)　emerge.　We　find　that,　the　SiC/turbostratic　graphite　interfaces　and　the　SiC　HAGBs　are　more　effective　in　suppressing　the　growth　of　SiC　grains　than　the　SiC/a-SiOC　interfaces　on　a　basis　of　extensive　TEM　characterization.　Aberration　corrected　TEM　reveals　marked　differences　in　the　atomic　structures　of　those　three　interfaces,　shedding　light　on　how　the　interfacial　structures　affect　grain　growth　of　SiC　fibers.