Nuclear magnetic resonance (NMR) spectra have been used to investigate the spinodal phase separation of sodium borosilicate glasses in the early stage, in which the phase separation is induced by rapidly quenching as opposed to heat treatment at a constant temperature. The experimental results show that the spinodal phase separation occurs under the quenching rates of 10(6) and 10 Ws. Although the domain size of the phase separation increases with decreasing quenching rate, B-11 NMR spectra and Si-29 MAS NMR spectra do not vary with the change of the domain size of the spinodal phase separation resulting from oxygen diffusion on quenching. The structure-analysis by NMR measurement indicates that the oxygen diffusion is controlled by a vacancy mechanism. On the basis of these NMR data, a structural model in the initial stage of the spinodal phase separation is proposed. The model suggests that the oxygen vacancy migrates by rotating itself to neighboring-bridged oxygen and reacting with the neighboring-bridged oxygen while the coordination number of the boron and silicon does not change. By the rotation and reaction, borons are connected together along the motion trace of the vacancy. According to the model, double-boron-group chains and triborate-group chains with high connectivity may be formed on rapidly quenched sodium borosilicate glasses.