The structures of two glucans, WGEW and AGEW, isolated from Gastrodia elata Bl. were elucidated using monosaccharide composition analysis by gas chromatography (GC), methylation analysis by gas chromatography-mass spectrometry (GC-MS) and nuclear magnetic resonance (NMR) spectroscopy. Their structures were deduced as an alpha-D-(1-->4)-glucan with an alpha-(1-->4) linked branch attached to O-6 branch points with different branch degrees. Their sulfate derivatives with distinct degrees of substitution (DS) were prepared. The substitution position was assigned to O-6 according to the (13)C NMR spectra. All sulfated derivatives showed strong anti-dengue virus bioactivities. The structure-activity relationships (SAR) between the polysaccharides and their sulfated derivatives were also investigated. Results showed that the higher the DS is, the more potent the impact on the dengue virus infection would be.