The　influence　of　the　expansion　device　on　the　reverse　cycle　defrost　performance　in　an　air-to-water　heat　pump　was　investigated　experimentally.　Experiments　were　conducted　on　a　55　kW　unitary　air-to-water　heat　pump.　A　copper　tube　with　an　outer　diameter　of　22　mm　and　the　thermal　expansion　valve　(TEV)　were　used　as　the　expansion　devices　for　the　defrost　cycle　respectively.　It　is　shown　that　the　defrost　time　for　the　copper　tube　test　is　40　seconds　less　than　that　for　TEV　test.　The　melting　time　and　the　draining　time　for　the　copper　tube　test　are　34　seconds　and　10　seconds　decreasement　respectively,　but　defrost　initiation　time　increases　4　s.　Due　to　the　small　inner　volume　of　the　plate　heat　exchanger　and　the　larger　refrigerant　flow　rate　during　the　defrost　termination　and　recovery　time,　the　discharge　pressure　had　the　sudden　increase　of　488.7　kPa　for　copper　tube　test　and　389.9　kPa　for　TEV　test.　In　addition,　the　discharge　pressure　for　the　copper　tube　test　was　453.8　kPa　higher　at　the　end　of　draining　time　than　that　for　TEV　test.　So　the　peak　of　discharge　pressure　for　the　copper　tube　test　was　2495.6　kPa　at　the　defrost　termination　and　recovery　time,　which　was　close　to　the　discharge　protection　value　(2550　kPa).