This　work　aims　to　elucidate　the　microstructure　characteristics　of　each　phase　in　carbidic　austempered　ductile　iron　and　the　nucleation　and　growth　of　acicular　ferrite　in　an　austempering　temperature　range　of　250-450　degrees　C.　The　research　results　showed　that　the　carbides　(Fe,　Cr)(3)C　obtained　by　the　eutectic　reaction　had　high　indentation　hardness　and　indentation　modulus,　and　the　interface　between　acicular　ferrite　and　eutectic　carbides　belonged　to　incoherent　boundary.　Since　the　nucleation　driving　force　of　acicular　ferrite　increased　with　an　increase　in　temperature　difference　(Delta　T),　the　size　of　acicular　ferrite　gradually　decreased　with　the　decrease　in　isothermal　temperature.　When　the　austempering　temperature　was　above　400　degrees　C,　acicular　ferrite　preferentially　nucleated　around　the　graphite　nodules　and　inside　grains.　However,　acicular　ferrite　preferentially　nucleated　around　the　graphite　nodules　and　the　eutectic　carbides　at　temperature　below　400　degrees　C.　Additionally,　a　fine　twin　structure　had　been　found　inside　some　acicular　ferrite　after　austempering　at　a　low　temperature　(250　degrees　C).