A theoretical study of the non canonical base pair, Guanine-Guanine (G-G) has been carried out in the frame of molecular orbital theory using density functional theory (DFT). The Becke three parameter hybrid including correlation functional that contains local and non-local terms (B3LYP) was used thoroughly. The 6-31G(d,p) basis set was employed to obtain the optimized geometry and energy of the non-Watson-Crick pair formed between two molecules of guanine. The results compare well with the properties of other base pairs of biological interest. At the DFT level the non-planarity of the pair is evident. The two guanines molecules are bent by ca 12 degrees and buckled. The interaction energy corrected by BSSE is equal to -10.7 kcal / mol, similar to that calculated for other pairs involving bases of nucleic acids.