Nanowires can be used for devices based on thermoelectrics and to convert the heat into electrical current. The material design challenge here is to create the materials where the conduction of heat would be kept low, and the electrical conductivity will be high. It was reported that the thermal conductivity, which is related to the phonon propagation, can be strongly reduced in rough Si nanowires that are 20¿300 nm in diameter. The goal of this proposal is the investigation of the phonon propagation and the effects of the confinement in Si nanowires using a TOF spectrometer MAPS. The results will help to understand the mechanism of the thermal transport in nanostructures, which in addition to the fundamental interest has practical implications in the design and performance of modern microelectronic devices.