Since 1971, the amount of substance mole has served as the definition of one of the seven fundamental units in the International System of Units(SI). One mole is defined as the atomic constant of 0.012 kg of the 12C carbon atom, so it is correlated with the definition of the mass kilogram. With the redefinition of the latter based on Planck's constant, the mole will be redefined using Avogadro's constant. This paper describes the principle of the new definition and its experimental determination. These results are of great significance for physics, chemistry and metrology.