Abstract: The expressing way of cotton yarn linear density in China had been changed form English count into tex system.But there are still some countries and areas using the English count for cotton yarn.A cotton yarn count conversion factor between the English count and the tex system may be used in business with some foreign countries and areas.The conversion factor is 583.1 or 590.5 in Chinese literature published.The value of the conversion factor appears differently.Aiming at two different values of the conversion factor in current literature,the formula for calculating the factor is deduced.After analyzing the source and the application situation of the two values,it is pointed out that the value 583.1 has no use-value for textile enterprises.The value 590.5(590.55,more accurate) should be uniformly used in textile filed.The value 583.1 should be avoided to using in international bid and trade activities.