We report the electrical transport properties of several small fullerenes sandwiched between two metallic electrodes and modulated by a gate electrode, calculated using an orthodox theory. The relationship between the charging energies and the structures of fullerenes is obtained. Bond types and molecular curvatures are observed to be important in determining charging energies. The calculated current-voltage and conductance characteristics are related to the electronic structures of fullerenes near energy gaps and are adjustable by the applied gate voltages, indicating that molecular devices with different functions can be designed by selecting fullerenes with proper bond types and curvatures, and by changing the applied gate voltages.