We present size-tunable assemblies of peptide nanowires and study their liquid crystalline phase behavior. An aromatic peptide molecule of diphenylalanine was rapidly assembled into one-dimensional nanowires, whose sizes were tunable according to the initial concentration of diphenylalanine in the preparing solution. The stable dispersion of peptide nanowires in an organic solvent exhibited colloidal nematic liquid crystalline phases. The liquid crystalline phase transition was governed by the repulsive electrostatic interaction among peptide nanowires as well as their shape anisotropy and polydispersity. Our work provides an interesting model system to investigate the liquid crystalline phase behaviors of a biomolecular assembly in terms of its assembled morphology and intermolecular interactions.