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Thermal conductivity of nanocrystalline materials is of quite general interest due to its significant technological importance in many practical applications of such materials,including bioMEMs1,thermal barrier coatings2 and micro-/nanoelectromechanical devices3,etc.In this paper,a universal equation for describing effective thermal conductivity(ETC)of nanocrystalline materials has been deduced,so that the ETC can be easily obtained from its grain size,single crystal thermal conductivity,single crystal phonon mean free path,and the Kaptiza thermal resistance.In addition,the relative importance between grain boundaries(GBs)and size effects on the ETC of nanocrystalline diamond at 300 K has been studied.It has been demonstrated that with increasing grain size,both GBs and size effects become weaker,while size effects become stronger on thermal conductivity than GBs effects.