Effects of retained compression strength on the collapsibility of CO₂mold sand using sodium silicate were studied. The results obtained from the experiment are summurized as follows; 1) The sand mixtures Increased their compression strength and retained compression strength when content of sodium silicate is high or mole ratio of sodium silicate is high. 2) Increase of retained strength has a maximum value at temperatures about 200℃. When the sample reached 800℃, the binder bridge are homogeneous The retained strength is increased 3) Decrease of retained strength at temperatures over 200℃ is caused by pore formation and additives of seacoal markedly accelerated pore formations.