Top-seeded melt growth is an effective method to fabricate REBaCuO large single domain superconducting bulks. It is a key factor to choose a right seed for preparing single domain SmBaCuO bulks which has a higher melting point. Six kinds of polydomain Nd-Ba-Cu-Mg-O bulk samples doped with MgO powders of 0.2%, 0.5%, 1%, 2%, 5% and 10% (mass fraction) were prepared through spontaneous nucleation with NdBaCuO as the substrate, and then near single domain SmBaCuO bulks were successfully prepared using the NdBaCuO with 1% and 10% MgO as seeds. The effect of MgO doping content on the melting temperature of NdBaCuO precursor powders, the saturation doping content of MgO powders in NdBaCuO substrate and characteristics of Nd-Ba-Cu-Mg-O seeds were investigated by DTA, XRD and OM. Results show that MgO powder doped in NdBaCuO can significantly increase the melting temperature of NdBaCuO precursor powder. The saturation value of MgO powders doping in NdBCO is about 1%. MgO-doping between 1% and 10% does not change the basic crystal structure of Nd-123 phase