Structural change of Cytochrome c peroxidase (CcP) due to interaction with lysine peptides (Lysptds) has been studied by absorption spectra and measurements on electron transfer between cytochrome c (cyt c) and CcP in the presence of Lysptd. Peaks were observed in the difference absorption spectrum of CcP between in the presence and absence of Lysptds, demonstrating a structural perturbation of CcP, at least at its heme site, on interaction with Lysptd. The interaction between CcP and Lysptd was electrostatic, since no significant peak was detected in the difference absorption spectrum when 100 mM of NaCl was added to the solution. Lysptds competitively inhibited electron transfer from cyt c to CcP, which indicated that they interacted with CcP at the same site as cyt c and would be models of the CcP interacting site of cyt c.