We explore the mechanism of the isospin splitting of neutron and proton effective masses in neutronrich nuclear matter within the framework of the Brueckner-Hartree-Fock (BHF) approach. First we find that the neutron-proton effective mass splitting in neutron-rich matter is dominated by the nonlocality of the microscopic s. p. potentials in spatial space,i. e.,by the isospin splitting of the k-mass shown in Fig. We explore the mechanism of the isospin splitting of neutron and proton effective masses in neutronrich nuclear matter within the framework of the Brueckner-Hartree-Fock (BHF) approach. First we find that the neutron-proton effective mass splitting in neutron-rich matter is dominated by the nonlocality of the microscopic sp potentials in spatial space, ie, by the isospin splitting of the k-mass shown in Fig.