In Northwest Hu’nan Province,there are kinds of valleys,which are firstly classified into three gradients,i.e.“U” (e.g.Mengdonghe River Valley),“V” (e.g.Dehang Valley) and “” (e.g.Xiaoxi Valley),according to the included angle between slope and base of valley,width of its base and runoff in this paper.Regarding the three valleys mentioned above as our study regions,we compared seed plant diversity composition and flora constituent structure among these different habitat conditions,and then revealed the characteristic and regularity of plant component corresponding to the alteration of gradient of valley.The detailed results are as follows:(1)Species composition shows obvious difference in three gradients of valley,between which the comparability coefficients get up to more than 80% and 70% at the family and genus levels respectively,while only about 50% at species rank.(2)Along with the gradient of valleys weakening,the portion of tropical component tends to decrease,accounting for 54.56% (Mengdonghe River),46.44% (Dehang) and 44.11% (Xiaoxi).Meanwhile the temperate component is with a trend of ascent of 40.00%,50.76% and 50.88%,respectively.Through this study,we find that all of the three valleys are both a refuge for relict taxa and a cradle for breeding new species although there are certain gradient and difference among them.Also,they are distinguished areas where seed plants distribute in Northwest Hu’nan Province.