Transmission　electron　microscopy　is　utilized　to　investigate　the　microstructure　development　of　iron-containing　SiC　fibers　as　a　function　of　their　annealing　temperatures.　Compositional　and　structural　gradients　along　the　radial　directions　start　to　form　within　SiC　fibers　fabricated　at　temperatures　above　1200　degrees　C.　The　graded　microstructures　are　associated　with　the　outward　diffusion　of　iron-rich　particles　along　the　radial　direction.　In-situ　heating　TEM　experiments　suggest　that　the　iron-rich　particles　within　SiC　fibers　are　highly　mobile　at　elevated　temperatures.　Therefore,　we　propose　that　the　graded　microstructures　along　the　radial　directions　are　the　synergetic　effects　of　iron　catalyzation　and　outgassing,　namely,　iron-rich　particles　are　progressively　driven　to　the　surface　of　SiC　fibers　and　in　the　trajectory　of　iron-rich　particles,　the　decomposition　of　the　amorphous　SiCxOy　phase　is　accelerated,　promoting　the　growth　of　SiC　grains.　The　current　study　emphasizes　the　important　role　of　the　outgassing　process　to　the　microstructure　evolution　of　SiC　fibers.　Our　proposed　mechanism　can　be　extended　to　understand　the　microstructural　evolution　of　SiC　fibers　that　use　metallic　additives　as　sintering　aids.