Thailandiol ( 1), gardenolic acid A ( 2), quadrangularic acid E ( 3) and 3beta-hydroxy-5alpha-cycloart-24(31)-en-28-oic acid ( 4) have been isolated from the leaves and twigs of Gardenia thailandica Tirveng (order: Rubiales; family: Rubiaceae). In addition, 5-hydroxy-7,2',3',4',5',6'-hexamethoxyflavone ( 5), 5,7-dihydroxy-2',3', 4',5',6'-pentamethoxyflavone ( 6), 5-hydroxy-7,2',3',4',5'-pentamethoxyflavone ( 7) and 5,7-dihydroxy-2',3',4',5'-tetramethoxyflavone ( 8) were also isolated from the same source. The structures were elucidated by spectroscopic methods. Crude extracts and compounds 1 - 4 displayed anti-HIV-1 activities as determined by using the (Delta)(Tat/Rev)MC99 virus and 1A2 cell line system. The EC (50) values determined by the syncytium assay ranged from < 7.8 to 110 microg/mL. They also exhibited moderate to high activities in reverse transcriptase (RT) assay; the IC (50) values of compounds 1 - 4, ranged from < 22.5 to 156.8 microg/mL.