The critical cooling rate required for the formation of glass is one of the important characteristics for silicate slag. The critical cooling rates have been measured for R₂O (R=Li, Na and K)-SiO₂ and R'O (R'=Ca, Sr and Ba)-SiO₂ systems by the combined use of the hot-thermocouple method and an ordinary melt-quenching method. In all systems, the minima in critical cooling rate corresponded with the composition range of low liquidus temperatures near the eutectics; the critical cooling rates decreased with increasing viscosities at the liquidus temperatures. The measured critical cooling rates were similar to those predicted by the Uhlmann equation and Ota equation. Some factors affecting the critical cooling rate were discussed.