One way of producing fine-grained materials is the extrusion method. The resulting extruded materials have a fine structure, texture, low thermal conductivity, a high mechanical strength and resistance to thermal influences. It was established experimentally that the characteristics of the extruded thermoelectric material are substantially influenced by factors such as pressure and temperature of the extrusion, the size of the starting powder particles, the temperature and annealing time for obtained thermoelectric material.   The present work is devoted to the influence of grain size on the electrical and thermal properties              of extruded samples of Bi0.5Sb1.5Te3 solid solution. The extruded samples were prepared from                  the synthesized material powders with grain size 1; 30; 50 m. We investigated in the temperature range of 80-300 K coefficients of electrical conductivity (σ) and the thermal conductivity (χ)                    of the extruded samples with different grain size and for comparison - single crystalline samples.   It was found that with decreasing grain size values coefficients of electrical conductivity (σ) and the thermal conductivity (χ)  decrease. Values of  χ  for fine-grained extruded samples over the entire temperature range are  2 times lower values of those for single crystals of the same solid solution.   Thermal conductivity is the sum of the electron thermal conductivity  е and thermal conductivity  of phonons  ph:  =  е +  ph. Lattice thermal conductivity can be reduced through phonon scattering on point defects and grain boundaries. Grain boundaries have a significant effect on the properties of solids. In polycrystalline samples grain boundaries and the grain surface layers always contain            a large number of defects [1-3], which distort the lattice and effectively scatter phonons, increasing the thermal resistance of the sample. In semiconductor solid solutions the greatest contribution                to the thermal conductivity provide long-wave phonons, as short-wavelength phonons are strongly scattered at existing in a large number point defects [4].   Potential barriers created by grain boundaries significantly reduces the carrier mobility, which leads to an effective increase of specific resistance of the semiconductor. Thus, by lowering grain sizes one can significantly reduce the lattice thermal conductivity, which are just observed in the investigated samples. Calculations showed that for the samples studied the electronic part of the thermal conductivity is not more than 22% of the total thermal conductivity. Therefore it can be assumed that the reduction in the total thermal conductivity is due mainly to a decrease in the lattice thermal conductivity.   At the same time, the smaller the dimensions of the crystallites (grains) the greater the probability of phonon scattering at the boundaries, and the lower the thermal conductivity. Grinding grains also leads to a decrease in electrical conductivity. Resistance of the inter-granular layers greater-than resistance of the grain volume, and an increase in the number of inter-crystallite layers with decreasing grain size leads to a decrease in electrical conductivity of finely structured samples.