Viscosities of molten RE-Mg-Si-O-N (RE=Y, Gd, Nd and La) glasses have been measured using rotating bob viscometer with a gas tight furnace at elevated temperature (~1873 K). Moreover, structural characterizations of these quenched vitreous samples have been investigated using solid state 29Si MAS-NMR, which would resolve the relationship between the viscosity of high temperature melts and network structure of RE-Mg-Si-O-N systems. The viscosities of molten RE-Mg-Si-O-N glasses exponentially increased with nitrogen content. 29Si MAS-NMR spectra of RE-Mg-Si-O-N (RE=Y and La) glasses revealed that content of silicon-oxynitride species, like SiO3N, increased with nitrogen content, which indicates that nitrogen clearly modifies the glass network structure. Depending on cationic radius of rare-earth elements, Y was found to be more effective in silicon-oxynitride species formation than La, which are consistent with the results of viscosity measurement of molten RE-Mg-Si-O-N glasses at elevated temperature (~1873 K).