A highly thermostable alpha-glucosidase (E C.3.2.1.20) from an extreme thermophile, Thermus thermophilus HB 8, was purified to homogeneous by ammonium sulfate fractionation, DEAE-cellulose chromatography and preparative slab gel electrophoresis. The enzyme was purified 17 fold with 21% recovery of activity. The enzyme had a molecular weight of 67000 by SDS-PAGE. The isoelectric point was pH4.5 by IEF on PAG. The enzyme hydrolized p-nitrophenyl-alpha-glucoside (PN-PG), sucrose and maltose, but not cellobiose, melibiose and soluble starch. The km value for PNPG was 0.4mmol/L, the Vmax was 0.29 mumol.min-1.mg-1. The enzyme exhibited high thermostability. After incubation at 90 degrees C for 10 h in 50 mmol/L acetate buffer pH 5.8, the enzyme retained 90% of its original activity. The half-live (t1/2) at 95 degrees C was 108 min. The enzyme was activated by Mg2+, Mn2+, Ca2+, Ba2+ and strongly inhibited by Hg2+, Cu2+. Modification of the enzyme by EDC or DEPC led to complete loss of activity, which suggests that carboxyl group(s) and histidine residue(s) are essential for activity of alpha-glucosidase.