Fine grained samples with a nominal composition of Bi_0.5Sb_1.5Te_x (x=3.0 to 3.4) were prepared by mechanical alloying (MA). Average grain sizes were found to range from 1.0 to 2.1 μm. Tellurium precipitation at x=3.1 was observed by scanning electron microscopy (SEM), X-ray diffraction and differential thermal analysis (DTA). Electrical conductivity as well as Hall and Seebeck coefficients were measured from 300 to 470 K. Using the relationship between Hall mobility and temperature, carrier scattering factors of the Bi_0.5Sb_1.5Te_x samples were estimated to range from r=−0.9 to −2.0. The thermal conductivities of the samples were found to range from 0.94 to 1.00 Wm⁻¹K⁻¹ at room temperature. Excess tellurium was recognized to be responsible for the decrease in thermal conductivity. The Lorenz number and the carrier component of thermal conductivity were calculated from the scattering factor and the Fermi integral. The phonon component of thermal conductivity was dominant in the Bi_0.5Sb_1.5Te_x samples. The Seebeck coefficient of the Bi_0.5Sb_1.5Te_x samples was higher than that from the available data which means that the absolute scattering factor of the Bi_0.5Sb_1.5Te_x samples was large. A maximum figure of merit of 7.02×10⁻³ K⁻¹ was obtained at 310 K for the Bi_0.5Sb_1.5Te_3.1 sample. This high figure of merit results from an enhanced Seebeck coefficient that is due to an increased absolute scattering factor. High thermoelectric performance can, therefore, be established by a reduction in the phonon component of the thermal conductivity and also by an enhanced Seebeck coefficient, which results from an increased absolute scattering factor for finely grained Bi_0.5Sb_1.5Te_x samples that were prepared by MA.