Abstract: The paper reviewed the concepts,sustained metabolic rate (SusMR) and sustained metabolic scope (SusMS),and four hypotheses on its limit factors were given,I.e.food availability hypothesis,peripheral limit hypothesis,central limit hypothesis and symmorphosis hypothesis.Besides,SusMRs evolutionary reason was analyzed with the methods and the conclusions.The relationship between SusMR and SusMS and RMR is that RMR increased at high SusMR,and the mechanism were concluded.SusMR has an important role on animal life.In the end,four future directions for this field were given.