The chemical structure studies on an important related substance of voglibose have been carried out using NMR spectroscopy and single crystal X-ray crystallography. For the structure identification study, hydrochloride monohydrate of this compound was isolated and purified. Its molecular structure was characterized and analyzed using 1D (1H NMR, 13C NMR, and DEPT) and 2D (1H,1H-COSY, HSQC, HMBC, TOCSY, and ROESY) NMR spectra. The signal crystal structure was detected by X-ray crystallography. All results indicated this related substance of voglibose was C17H27NO7·HCl·H2O, 5,6-dideoxy-4-benzyl-5-{[2-hydroxy-1-(hydroxymethyl)ethy]amino}-1-C-(hydroxymethyl)-D-epi-inositol hydrochloride monohydrate.