Diphenylalanine (FF) peptide nanotubes (PNTs) are the unique class of self-assembled biomaterials because of wide range of useful properties including biocompatibility, mechanical rigidity, ferroelectricity, chemical and temperature stability. In this work we report results of investigation of FF peptide nanotubes and examined their optical second harmonic generation (SHG). SHG is very powerful tool for investigation of the structural properties1, including different types of phase transitions2,3 domain structure4, electronic structure5, etc. The studied PNTs were obtained by a self-assembly process of the diphenylalanine NH2-Phe-Phe-COOH (FF) peptide monomers deposited as aqua solution onto a Si and Au substrate. We demonstrate highly efficient two-photon processes of optical SHG and two-photon luminescence in peptide nanotubes (PNTs) excited with the femtosecond Ti:Sapphire laser. Our measurements revealed some evidence of ferroelectric behavior in FF peptide nanotubes at room temperature. SHG signal decrease with temperature. Signal disappeared at 140-150 ºC signaling possible ferroelectric-paralectric transition. In addition we describe the connection between the protein structure and its optical properties (Figure 1). On left panel we can see linear-optical image of PNTs. SHG image of the same region is on the right panel. There is correlation between linear and nonlinear optical images. Tubes with the orientation perpendicular to the polarization of the incident light have the higher SHG intensity.figureFigure 1. Images of peptide nanotubes (a) linear-optical image, (b)nonlinear-optical image.