A　study　is　presented　on　the　characteristics　of　the　air　source　heat　pump　(ASHP)　under　two　kinds　of　typical　mal-defrost　phenomena.　The　definition　of　the　＂Mal-defrost　phenomena＂　is　put　forward,　and　the　possible　origins　of　their　occurrence　are　analyzed.　It　is　concluded　that　the　current　defrost　strategies　may　cause　mal-defrost　phenomena　due　to　the　lack　of　the　direct　control　information,　frost　thickness.　Transient　characteristics　of　the　tested　ASHP　under　these　mal-defrost　phenomena　are　investigated　by　field　tests.　Instant　frosting　images　are　captured　to　record　the　dynamic　frosting　process.　In　the　first　phenomenon　of　mal-defrost,　the　defrost　control　is　carried　out　over　an　hour　after　a　＂critical＂　level　of　frosting　has　been　reached.　It　reduces　the　COP　and　heating　capacity　by　about　17.4%　and　29%,　respectively.　In　the　second　phenomenon　of　mal-defrost,　three　defrost　operations　occur　while　almost　no　frost　is　observed　on　the　heat　exchanger　during　6　h　testing　period.　It　causes　4.2%　decrease　for　the　heating　efficiency.　(C)　2011　Elsevier　Ltd.　All　rights　reserved.