In order to investigate whether endohedral He-doping is possible or not in CL-20, a density functional treatment has been carried out at the levels of B3LYP/6-31++G(d,p) and B3PW91/6-31++G(d,p). Some physicochemical and quantum chemical properties of the helium-doped CL-20 (He@CL-20) are compared with the respective values of the parent explosive CL-20. The helium doping caused swelling of CL-20 cage but no bond rupture occurred. Doped helium acquired some positive charge.