On the basis of the G·matrix calculation with the Reid soft core potential, the effective two· nucleon interaction ( V) of the central force type in asymm~tric nuclear matter is constructed. It is expressed in the five· range Gaussian and depends on the two· nucleon state, the density and the asymmetry parameter (a). This effective interaction is expected to be useful in the study on the properties of neutron star matter where protons admix with neutrons. The proton fraction of cold neutron star matter is investigated as an example and the importance of the a-dependence of V is stressed. Possible approximation for the a-dependence of the potential energy is also discussed.