The　two-phase　flow　in　the　primary　converging-diverging　nozzle　of　a　transcritical　CO2　ejector　was　investigated　by　a　visualization　measurement　experiment　using　the　direct　photography　method.　The　transition　of　the　phase　change　position　in　the　primary　nozzle　was　discovered　experimentally　and　analyzed　theoretically　based　on　the　isentropic　expansion　process.　The　pressure　distribution　along　the　primary　nozzle　was　measured　by　the　pressure　sensors　simultaneously　under　different　operating　conditions.　The　expansion　state　of　the　primary　flow　and　the　nozzle　outlet　quality　were　studied　by　comparing　the　nozzle　outlet　pressure　with　the　suction　flow　inlet　pressure.　The　visualization　images　revealed　that　the　phase　change　could　start　after　or　before　the　throat　according　to　the　operating　conditions,　which　was　consistent　with　the　isentropic　analysis　results.　In　addition,　the　phase　change　position　moved　upwards　when　the　primary　flow　inlet　pressure　and　temperature　decreased　simultaneously.　The　pressure　measurement　results　found　that　the　nozzle　outlet　pressure　was　obviously　higher　than　the　suction　flow　inlet　pressure　in　the　experiment,　which　indicated　that　the　primary　flow　was　under-expanded　in　the　primary　nozzle.　The　nozzle　outlet　quality　calculated　with　the　nozzle　outlet　pressure　was　less　than　that　calculated　with　the　suction　flow　inlet　pressure.　The　visualization　experiment　can　be　used　to　improve　the　CO2　ejector　geometry　and　provide　useful　data　for　CO2　ejector　model　validation　to　better　interpret　the　internal　complex　flow　phenomena.