Nanocomposites composed of thermoplastic polyurethane block copolymer (PU) and multi-walled carbon nanotube (CNT) were prepared by melt-compounding, and their mechanical and thermal properties were investigated along with electrical conductivity. Modulus of composites increased proportionally with increasing CNT content breaking stress tended to decrease except 1% CNT, and strain-at-break decreased gradually. However, composites using sonicated nanotube showed different mechanical behavior, and especially, their strain-at-break increased largely due to better dispersion of nanotube in PU. It was considered that tensile properties of composites were dependent on CNT content, dispersion of nanotubes in matrix, and phase separation between hard and soft segments of PU due to nanotube addition. With increasing CNT content, glass transition temperature and maximum loss tangent decreased and thermal stability improved. Also electrical conductivity increased to 10$^{-4}$ S/cm for the 3% CNT composite.