Full geometrical optimizations of 1,1-dimethylhydrazine, (CH₃)₂NNH₂, have been carried out by quantum-mechanical methods RHF, DFT/B3LYP and MP2 respectively. The optimal geometric structure parameters have been obtained. The effects of basis set are investigated. The predicted structure parameters are compared to the experimental ones also. Except two predicted parameters, C—N—N—H dihedral angle and C—H bond length, show smaller values, the others agree well with the experimental ones. All of these calculations indicate a gauche conformation. The predicted N—N bond length is about 0.143 nm and C—N bond length is about 0.147 nm. The bond angles of N—N—C and C—N—C are about 112° and 113.8° respectively. The C—N—N—H dihedral angle is about 86°。The calculations at 6-31G basis set can get enough precisions.