A novel Lachnum sp. exopolysaccharide (LEP-2a) was separated and purified by DEAE-52 cellulose and Sephadex G-100 chromatographic columns, with relative molecular weight of 2.3 x 10(4) Da. One- and two-dimensional nuclear magnetic resonance analysis combined with methylation analysis results revealed that LEP-2a was a galactomannan with a backbone structure composed of alpha-(1 -> 3,4)-D-Manp, alpha-(1 -> 2)-D-Manp, alpha-(1 -> 2,6)-D-Manp and beta-(1 -> 3)-D-Galp residues, which was substituted at O-3,O-4, O-2, O-6 by branches. Side chains were composed of alpha-(1 -> 3,4)-D-Manp, alpha-(1 -> 2)-p-Manp, alpha-(1 -> 2,6)-D-Manp and beta-(1 -> 3)-D-Galp terminated by terminal alpha-D-Manp. Activities researches enlightened that LEP-2a could play a role in reducing blood glucose as one a-glucosidase inhibitor. (C) 2016 Published by Elsevier Ltd.