Abstract:Abstract Rarity and the degrees of threa tened plants are the basis of asscssnl-ent for the priority of conservation．However the concepts of rarity and the degrees of threatenedness are defined as directional description In home and abroad，that the standards are difficult for us to catch．In accordence with Individual ecology， population dynamic and community ecosystem，the patterns of range，the regional distribution，the spedes limitation of community，the important value of species In community and the population structure of spedes In community have been considered as the expressive messages of rarity and threatenedness．The strength of the messages expressionthen are marked by using a scale of range number between 5 and 1 according to the difference of environmental impact，and then a formu－lation of coefficience of threatenedness is suggested for assessing the categories of threatened spedes．