An eco-friendly and convenient preparation method for notoginsenoside ST-4 has been established by completely transforming vina-ginsenoside R-7 using a recombinant glycosidase hydrolyzing enzyme (HaGH03) from Herpetosiphon aurantiacus. This enzyme specifically hydrolyzed the glucose at the C-20 position but not the external xylose or two inner glucoses at position C-3. Protein sequence BLAST revealed that HaGH03, composed of 749 amino acids and presumptively listed as a member of the family 3 glycoside hydrolases, has highest identity (48 %) identity with a thermostable beta-glucosidase B, which was not known of any functions for ginsenoside transformation. The steady state kinetic parameters for purified HaGH03 measured against p-nitrophenyl beta-D-glucopyranoside and vina-ginsenoside R-7 were K (M) = 5.67 +/- 0.24 mu M and 0.59 +/- 0.23 mM, and k (cat) = 69.2 +/- 0.31/s and 2.15 +/- 0.46/min, respectively. HaGH03 converted 2.5 mg/mL of vina-ginsenoside R-7 to ST-4 with a molar yield of 100 % and a space-time yield of 104 mg/L/h in optimized conditions. These results underscore that HaGH03 has much potential for the effective preparation of target ginsenosides possessing valuable pharmacological activities. This is the first report identifying an enzyme that has the ability to transform vina-ginsenoside R-7 and provides an approach to preparing rare notoginsenoside ST-4.