The characteristics of hot air drying in the drying chamber of a sizing machine was numerically analyzed with respect to operating and ambient conditions. The results showed that the proposed model described Qualitatively the behavior of drying process of sized yarns in the drying chamber of a sizing machine well. The drying rate and moisture content of sized yarn were strongly dependent on the air temperature, humidity, and mass flow rate. From parametric studies, the optimal operating conditions were searched and obtained under given input and ambient conditions.