α-Mangostin, a major xanthone isolated from the pericarp of Garcinia mangostana, exhibits anti-inflammatory and antitumor effects. Its absolute bioavailability is low, with minimal oral absorption. In this article, a soft capsule, with vegetable oil as the dispersion matrix, was prepared to improve the bioavailability of α-mangostin. Its pharmacokinetics and tissue distribution were determined in rats. An HPLC assay was established to determine the concentration of α-mangostin in biological samples. The validated method was used successfully to support pharmacokinetic and tissue distribution studies of α-mangostin in rats after intravenous (i.v.) and oral administration. The pharmacokinetic study found the absolute bioavailabilities of low, medium and high doses were 61.1, 51.5 and 42.5 %, respectively, indicating that the absolute bioavailability was effectively improved.