The antifungal activities of methanolic extracts from Terminalia nigrovenulosa bark (TNB) was investigated for effects on the initial growth of mycelia against Fusarium solani. The ethyl acetate fraction separated from TNB demonstrated the highest antifungal activity against F. solani. The antifungal compound was isolated from TNB using silica gel column and Sephadex LH-20 chromatography combined with thin-layer chromatography and high performance liquid chromatography. Structural identification of the antifungal compound was conducted using (1)H NMR, (13)C NMR, and liquid chromatography-tandem mass spectrometry. The purified antifungal compound was gallic acid (GA) or 3,4,5-trihydroxy benzoic acid. Purified-GA possesses the high antifungal activity against F. solani, and that antifungal activity was dosage-dependent. The hyphae became collapsed and shrunken after 24 h incubation with GA (500 ppm). In pot experiments, the application of TNB crude extract was found to be effective in controlling the cucumber Fusarium root rot disease by enhancing activities of chitinase, peroxidase thereby promoting the growth of plants. The applied TNB extract significantly suppressed root rot disease compared to control. It resulted in 33, 75 and 81% disease suppression with 100, 500 and 1000 ppm of TNB crude extract, respectively. The study effectively demonstrated biological activities of the TNB extract, therefore suggesting the application of TNB for the control of soil-borne diseases of cucumber plants.