A　3D　particle-in-cell/Monte　Carlo　collision　model　is　used　to　investigate　the　streamer　discharge　inception　in　CO2　at　elevated　pressures　including　gaseous,　liquid　and　supercritical　phases.　Generation　of　free　electrons　is　a　prerequisite　for　initiation　and　development　of　positive　streamers.　Field　ionization　from　impurity　molecules　of　low　density　and　low　ionization　energy　is　assumed　as　the　source　of　primary　ionization　in　the　model.　The　field　dependent　generation　rate　of　electrons　is　calculated　by　Zener＇s　model.　Tree-like　streamer　with　filamentary　shape　presents　at　the　needle　tip　and　propagates　towards　the　plane　electrode　in　all　three　phases　of　CO2,　with　differing　in　length　and　propagation　speed.　Furthermore,　the　influence　of　different　parameters　such　as　applied　voltage,　electrode　tip　size,　and　ionizable　density　and　ionization　potential　of　impurity　molecules　on　the　evolution　of　streamer　discharge　in　supercritical　CO2　are　investigated.　Simulation　results　obtained　from　the　3D　model　agree　well　with　experimental　observations　in　the　literature,　which　show　that　field　ionization　might　be　one　of　possible　ionization　sources　in　pressurized　CO2　in　the　presence　of　high　electric　field.　©　2020　The　Authors.　High　Voltage　published　by　John　Wiley　&　Sons　Ltd　on　behalf　of　The　Institution　of　Engineering　and　Technology　and　China　Electric　Power　Research　Institute.