In the recent years all boron fullerene have been noticed as drug delivery system. Due to the fact that so far metal encapsulated fullerenes has not been investigated as a carrier of 6-Thioguanine(6-TG), therefore in this study, adsorption behavior of (TG) on the B40 and M@ B40 (M = Mg and K) fullerene was investigated through density functional theory (DFT). We found that the 6-TG interacts with the M@ B40 fullerene stronger than bare B40 fullerene in the gas and water phases. The calculated adsorption energies are about −14.2, –22.5 and −15.7 kcalmol−1for B40, Mg@B40 and K@ B40 respectively. Dipole moment of encapsulated complexes enhanced by 50 % and 80 % in the gas and water phase respectively. The calculated energy gap (Eg) and the other reactivity parameters indicates that, reactivity of B40 fullerene was enhanced in presence alkaline and alkaline earth metal. In the result, B40, Mg@ B40 and K@ B40 fullerenes have the potential of delivery of 6-TG anticancer drug.