Field　ionization　is　adopted　as　a　streamer　initiation　mechanism　in　highly　pressurized　CO2,　using　a　3D　particle　model.　The　field　ionization　is　implemented　into　the　3D　model　through　Zener＇s　model.　Streamer　evolution　is　classified　into　three　stages;　inception　cloud　formation,　main　streamer　channels,　and　subsequent　streamer　branching　independent　of　the　medium　state.　The　comparison　of　average　electron　density,　electric　field,　and　the　length　of　streamer　in　three　thermodynamic　states　of　CO2　is　presented.　Box　counting　method　is　used　to　calculate　the　fractal　value　(D)　for　positive　streamer　pattern　in　gaseous,　liquid,　and　supercritical　(SC)　CO2.　The　figures　of　streamer　discharge　obtained　from　the　3D　simulation　model　are　fractal　with　fractal　dimension　for　gaseous,　liquid　phase,　and　SC-　CO2　as　1.577,　1.578,　and　1.587.　The　results　of　fractal　analysis　of　positive　streamer　branching　in　this　paper　are　consistent　with　the　fractal　values　of　streamer　in　the　SC-CO2　through　experiments　in　literature.　©　2021　IEEE.