We investigated the binding nature of three peptides (inactive NB1 and active B1 and B3) to single-walled carbon nanotubes (SWCNTs) using a density functional tight-binding (DFTB) method with an empirical van der Waals force correction. We show that the three peptides could be spontaneously adsorbed to the carbon nanotube (CNT) surface through π−π and/or H−π stacking at physisorption distances and the geometric and π-electronic structures of SWCNTs remain basically undamaged upon the adsorption. We also investigated the diameter and chirality dependence of binding energies. The calculated results are consistent with experimental observation, and we found that aromatic residues, such as His and Trp, are the keys in determining peptide/CNT binding. In addition, our calculations predict that noncovalent modification of SWCNTs by the active peptides might increase the electron transfer capabilities of SWCNTs.