[en] The effects of Nd-doping on the transport and magnetic properties of La_2/3-yNd_ySr_1/3MnO₃ (y=0, 1/3, 2/3) are studied theoretically by using the generalized-gradient-corrected full-potential method. In order to investigate the coupling between Nd and Mn, the electronic structures of La_2/3-yNd_ySr_1/3MnO₃ with ferromagnetic (FM) and antiferromagnetic (AFM) arrangements of Nd and Mn sublattices are calculated. The calculation for FM La_2/3-yNd_ySr_1/3MnO₃ yields a half-metallic band structure, while the ferrimagnetic (FiM) system is found to have a metallic character. Hybridization of Nd 4f, Mn 3d, and O 2p bands around Fermi level (E_F) is observed, suggesting the coupling between Nd and Mn is mediated by O 2p carriers. The qualitative features of transport and magnetic properties of such a two-spin system can be interpreted in terms of half-metallic FM domains being mixed up with metallic FiM domains. The proportion of FM domains varying with Nd-doping concentration has strong influences on the magnetoresistance