One of the bottlenecks for bioproduction of butyric acid as bulk chemical is the difficulty in separating butyric acid from the fermentation broth, compared with the petroleum-based chemical synthesis method. In the present work, a novel separation methodology was developed based on an aqueous two-phase system with inorganic salts. Calcium chloride was screened out for effective separation of butyric acid from butyric acid-water-salt systems. Within appropriate concentration range of butyric acid and salt, butyric acid was enriched in the upper phase and most of calcium ions remained in the lower phase. This "salting out" effect is very efficient to separate butyric acid from the simulated butyrate fermentation broth, which consists of butyric acid and acetic acid with concentration ratio of 4 : 1, so that the final ratio of butyric acid/acetic acid in the upper phase is improved to 9.87. The aqueous two-phase system was used to separate butyric acid from the actual fermentation broth with satisfactory result.