Cyanide poisoning is expected to be antagonized by the administration of oxygen, when it is administered in combination with the conventional cyanide antidote, sodium thiosulfate. However, the antidotal efficacy and its exact mechanism of oxygen in cyanide poisoning is still a controversial one. To test the effect of oxygen on the antidotal action of thiosulfate in cyanide poisoning, author designed this study on the dose-mortality patterns for potassium cyanide in mice. Potency ratios derived from $LD_{50}$ values were compared in groups of mice treated with sodium thiosulfate alone and sodium thiosulfate with oxygen. These results indicated that oxygen enhances the antidotal effect of sodium thiosulfate, effectively. This fact demonstrates that oxygen is of importance in the treatment of cyanide poisoning.