Using cigarettes made of different bright tobaccos but with a constant fractional volume, following results were obtained. The rate of formation of CO was a more effective factor for the amount of CO in the mainstream smoke than the weight loss of cigarette during puffs. From a stepwise multiple regression analysis, it was found that the rate of formation of CO was mainly dependent on potassium content of tobacco and could be estimated from the content of potassium, total carbon and lignin. The rate of formation of CO increased with a rise in combustion temperature, which rose with a decrease inpotassium content of tobacco. Hence, it was concluded that the enhancement of the rate of formation of CO was due to the rise in burning temperature resulting from the decrease in potassium content of tobacco.