Modulation of the fibrillogenesis of amyloid peptide A beta(1-40) with the cationic gemini surfactant hexamethylene-1,6-bis(dodecyldimethylammonium bromide) (C12C6C12Br2) has been studied. Both UV-vis and AFM results show that C12C6C12Br2 monomers can promote the fibrillogenesis of A beta(1-40) while its micelles inhibit this process. The electrostatic/hydrophobic force balance plays important roles in determining the A beta(1-40) aggregation style and the secondary structures. When the surfactant positive charges are close to the A beta(1-40) negative charges in number, the hydrophobic interaction is highly enhanced in the system. Both the nucleation rate and the lateral association between fibrils are greatly promoted. However, when the surfactant positive charges are in excess of the A beta(1-40) negative charges, the electrostatic interaction is strengthened. In this case, the lateral association is inhibited and the (alpha-helix to beta-sheet transition in the secondary structure is prevented. Simultaneously, another assembly pathway is induced to give the amorphous aggregates. Moreover, the size and surface roughness of the A beta(1-40) aggregates also vary upon increasing C12C6C12Br2 concentration.