The effect of Na₂O on the behavior and mechanism of the precipitation of cuspidine from the CaO–SiO₂–CaF₂–Na₂O quaternary slags has been investigated by solid-state ¹⁹F nuclear magnetic resonance spectroscopy, differential thermal analysis and X-ray diffraction analysis. It is found that F⁻ predominantly bonds with Ca²⁺ rather than Na⁺ in the composition range of the commercial mould fluxes. This result indicates that the presence of Na₂O may not prevent the precipitation of cuspidine from the slags. The glass transformation temperature and the crystallization temperature of cuspidine decrease with increasing Na₂O content. The mass fraction of cuspidine in the samples heated at 973 K was obtained as a function of heat treatment time. It is found that the mass fraction of cuspidine significantly increases at the early stage of the heat treatment, but levels off later on. The mass fraction of cuspidine at the leveling-off is the largest for the sample with 4 mass% Na₂O, and is smaller as the Na₂O content is either smaller or larger than 4 mass%. It is also found that the incubation period is observed before the crystallization starts, and that the incubation period is shorter as the Na₂O content in the sample is larger.