The effects of add-on finish and some other process variables on mechanical and mass irregularity characteristics of air-jet spun polyester yarns have been studied using the Box-Behnken design. It is observed that the add-on finish plays a dominant role in influencing the yarn properties. The correlations between spinning parameters and mechanical properties are found to be highly significant. On the other hand, the yarn irregularity exhibits poor correlation with the process variables. Imperfection indices show a general ascending relationship with add-on finish and other process variables studied.