2-(3,4-Dihydroxyphenyl)-5,7-dihydroxy-3-[(2S,3R,4S,5R,6R)-3,4,5-trihydroxy-6-(hydroxymethyl) oxan-2-yl]oxychromen-4-one is known as quercetin 3-D-galactoside (Q3G) (C21H20O12). Under theoretical study of Q3G and the potential energy surfaces of the molecule, about 12 bonds were obtained through 360o full scanning by 6o increments. All conformers were optimized by using (DFT/B3LYP and DFT/LSDA) methods with the 6-31G (d,p) basis set in gas and ethanol phases. Twenty-one and 25 conformers of Q3G in gas phase were obtained. The most stable conformers (Q3G-G1 in gas phase and Q3G-E1 in ethanol phase) were re-optimized by two different basis sets. HOMO-LUMO energy gaps of Q3G molecule were calculated to be 4.96 for Q3G-G1 and 3.98 eV for Q3G-E1. UV spectra of Q3G were calculated using TD-DFT and ZINDO methods. Two strong transitions were obtained at 365.2 and 359.1 nm in gas and ethanol phases, respectively. The formations of hydrogen bonds were examined by natural bond orbital analysis, and three important intramolecular H-bonds were found. Normal coordinate analysis of Q3G-G1 was made, and all the bands of the vibrational spectra were interpreted by emphasizing intramolecular interactions.