The (Bi_0.5Sb_1.5)Te₃ samples were prepared by mechanical alloying followed by hot pressing. Grain sizes were controlled in a range from 1 to 10 μm by controlling the sintering temperature in a range from 623 to 773 K. A fine measurement system was constructed for thermal conductivity based on the static comparison method. The thermal conductivity of the sample was 1.397 W/mK. The system has an accuracy of less than 1% for 1.411 W/mK of the reference quartz. The thermal conductivity was 0.89 W/mK for the sample sintered at 623 K, where the grain size of 1.75 μm was measured by scanning electron microscopy (SEM). The thermal conductivity increased on the sample sintered at 673 K because of grain growth and decreased on those sintered at the temperatures from 673 to 773 K because the increase of pore size caused to decrease thermal conductivity. The increase of thermal conductivity for the samples sintered at temperatures above 773 K was affected by the increase of carrier concentration.