Abstract：The DFT (B3LYP/6-31＋G**) method was used to study the conformations and isomerization of the isolated and monohydrated 2',3'-dideoxy-2',3'-didehydroguanosine (D4G) molecules in gas phase. The effects of one-water molecule on relative stabilities, conformational parameters and nature charges of D4G were analyzed. The results indicate that there are 8 stable conformers for D4G in gas phase. The conformer d4g-2 is the most stable form of all conformations in gas phase. The relative stability order is as follows: d4g-2＞d4g-1＞d4g-5＞d4g-3＞d4g-6＞d4g-4＞d4g-8＞d4g-7. The computational results are in good agreement with the available experimental ones. The addition of one-water molecule did not almost change the order of the relative stability and increased slightly the energy barriers of conformational isomerization of D4G. Hydrogen bonding played an important role in D4G conformations.