The extraction of propionic acid has been studied by liquid-liquid extraction with poly ethylene glycol /different salts of sulfate aqueous two-phase system (ATPS). In this study, a multilevel factorial design was applied to evaluate the effects of four independent variables (PEG molecular weight, type of salt, salt concentration, and temperature) on propionic acid extraction yield. The statistical significance of each factor was evaluated by an analysis of variance. The results indicated that the type and concentration of salt were the major contributing factors to propionic acid extraction. The PEG4000/ MnSO4 system was suitable for the extraction of propionic acid from dilute water.