Annuals 30-140(-200) cm tall. Roots yellow, fibrous. Stems erect, 2-6 mm in diam., branched. Basal leaves withered at anthesis. Inflorescences panicles of cymes, lax, spreading branched, many flowered, to 50 cm. Flowers 5merous. Filaments 5-6.5 mm; anthers ellipsoid, ca. 2.5 mm. Style indistinct; stigma lobes capitate. Capsules narrowly ovoid, to 2.3 cm. Seeds brown, globose; seed coat warty. Fl. and fr. Jun-Nov. It contains main chemicals sawertiamarine, mangeferin and amarogenitine. 1,5,8-trihydroxy-3-methoxyxanthone, 1-hydroxyl-2,3,5,7-tetramethoxyxanthone, 1-hydroxyl-3,5,8-trimethoxyxanthone, 1-hydroxyl-2,3,4,6-tetramethoxyxanthone, 1-hydroxyl-2,3,4,7-tetramethoxyxanthone, 1,8-dihydroxy-3,5-dimethoxyxanthone, 1,7-dihydroxy-3,8-dimethoxyxanthone, 1,3,5,8-tetrahydroxyxanthone, balanophonin, oleanolic acid, maslinic acid, and sumaresinolic acid. Swerilactones from Swertia mileensis showed anti-hepatitis B virus activity in vitro.