Abstract: We investigated the phonon and thermal properties of the silicon- and germanium-nanowires, covered by Si_xGe_1–x, plastic, diamond and SiO₂ shells as well as Si-based segmented nanowires, consisting of segments of different sizes and/or materials. Acoustic phonon energies were calculated in the framework of the face-centered cubic cell model of the lattice vibrations, while thermal conductivity was investigated in the framework of Boltzmann transport equation approach within the relaxation time approximation. It was shown, that claddings with higher (lower) sound velocity strongly affect the phonon energy spectra and increase (decrease) the average phonon group velocity in core nanowire. It was demonstrated, that redistribution of the phonon modes in Si/Ge and Si/SiO₂ segmented nanowires leads to a localization of the great amount of the phonon modes in nanowire segments, resulting in exclusion of such modes from the heat flow and suppression of the phonon thermal conduction (by a factor of 2–8) in comparison with generic silicon nanowires. Low values of the thermal conductivity of segmented nanowires make them prospective for thermoelectric and thermoinsulating applications.