Abstract: The effects of gsain size on the effective anisotropy constant ＜K_sh＞ between the exchange-coupled magnetically soft（α-Fe） and hard(Nd₂Fe₁₄B) nano-grains were investigated. Considering the existence of the interfacial exchange coupling between grains, the grain can be divided into two parts: inner part without interfacial exchange-coupling effects（uncoupled） and interfacial part with interfacial exchange coupling effects（coupled）. For the given soft grain size D_s, ＜K_sh＞ increases with the increasing hard grain size D_h, and then approaches the magnetic anisotropy of the hard phase. For the given D_h, ＜K_sh＞ increases with decreasing D_s. A large ＜K_eff＞ is predicted in nanocomposite for the hard grain size larger than 35 nm and the soft grain size about 10 nm.