A　theoretical　study　was　conducted　to　investigate　the　critical　heat　and　mass　transfer　characteristics　on　a　frosting　tube.　The　parameters,　Critical　Frost　Thickness　(CFT)　and　Neutral　Frost　Thickness　(NFT)　were　presented　for　the　first　time.　The　heat　transfer　was　found　to　increase　before　the　point　CFT,　and　decrease　to　the　initial　value　of　a　non-frosted　tube　at　the　point　NFT.　The　findings　provide　a　reasonable　explanation　for　the　heat　transfer　increase　for　a　fin-tube　heat　exchanger　at　the　early　stages　of　frost　formation.　By　theoretical　study,　the　values　of　CFT　and　NFT　were　impacted　by　the　following　five　parameters,　the　air　temperature　T-a,　humidity　phi,　velocity　V,　tube　wall　temperature　T-w　and　tube　outside　diameter　d(2).　The　influences　of　these　parameters　on　the　CFI＇　and　NFT　were　further　discussed.　By　sensitivity　analysis,　the　CFI＇　and　NFT　were　found　to　be　more　sensitive　to　the　parameters　of　phi,　V　and　d(2).　(C)　2013　Elsevier　Ltd.　All　rights　reserved.