In this study, the effects of vacuum steaming process parameters on physical properties, tensile properties and twist liveliness values of spun and filament 100% polyester yarns were investigated. For this purpose, yarns having different twisting coefficients and different numbers were twisted, and afterwards they were exposed to vacuum steaming at different temperatures and for different durations. Linear density, hairiness, unevenness values, tensile properties and twist liveliness of these yarns were measured before and after vacuum steaming. The results obtained were assessed by means of COSTAT and SPSS statistical analysis programs and variation analysis, and Student-Newman-Keuls tests were carried out at 5% (0.05) level of significance. The results of this study indicate that the effects of vacuum steaming temperature and duration on tensile properties, unevenness, hairiness and twist liveliness of spun polyester yarns, and tensile properties and twist liveliness of filament polyester yarns, are statistically significant.