We study the doping of a free C 60 molecule by potassium atoms. Ab initio calculations based on density functional theory indicate that K atoms are fairly strongly bound to the carbon cage. The electrical conduction of the structure is also enhanced after each successive doping of K. We also observe a downward shift of energy levels toward the Fermi energy with the increase of doping. These properties are very consistent with experimental results. The K-doped structure is promising for applications in molecular electronics.