Frost　build-up　blocks　the　gap　between　fins　and　decreases　the　airflow　rate,　and　the　heating　capacity　and　COP　of　air-source　heat　pump　become　lower.　The　reverse-cycle　defrosting　(RCD)　and　hot-gas　bypass　defrosting　HGBD　are　the　two　common　methods　of　frost　removal.　The　comparison　between　the　RCD　and　HGBD　methods　on　one　heat　pump　were　not　found　by　authors　up　to　now.　This　paper　compared　the　dynamic　characteristics　during　the　RCD　and　HGBD　periods　on　one　medium　air-to-water　heat　pump　of　55　kW.　The　feasibility　of　the　HGBD　method　without　heat　provided　to　evaporate　the　liquid　refrigerant　after　melting　frost,　which　was　supposed　in　the　danger　of　the　compressor　liquid　slugging,　was　especially　discussed.　The　results　showed　the　liquid　refrigerant　leaving　the　air　cooled　coils　for　the　HGBD　method　was　trapped　in　the　suction　accumulator　and　flashed　into　the　saturated　gas　by　the　compressor　pumping　action,　and　no　compressor　liquid　slugging　happened　in　our　tests.　The　HGBD　time　was　much　longer　than　the　RCD　time.　The　amenity　for　the　HGBD　method　was　better　than　that　for　the　RCD　method,　due　to　lower　refrigerant　noise,　smaller　indoor　temperature　fluctuation,　and　no　cold　blowing.　The　suction　superheat　and　discharge　superheat　for　the　HGBD　method　were　lower　than　those　for　the　RCD　method.　The　HGBD　could　overcome　the　main　disadvantages　of　the　RCD　method　and　deserved　the　further　investigation.　(C)　2008　Elsevier　Ltd.　All　rights　reserved.