An antimicrobial diterpene was isolated from Alpinia galanga in the screening for potentiators of phytochemical antibiotic action. The structure was elucidated by spectral data and identified as (E)-8 beta, 17-epoxylabd-12-ene-15, 16-dial (1). Diterpene 1 synergistically enhanced the antifungal activity of quercetin and chalcone against Candida albicans. Antifungal activity of 1 was reversed by unsaturated fatty acids. Protoplasts of C. albicans were lysed by 1. These results suggest that antifungal activity of 1 is due to a change of membrane permeability arising from membrane lipid alternation.