The sintered composite of Bi_0.5Sb_1.5Te₃ and ionic liquid, BMImTFSI, was synthesized by sintering the mixture of Bi_0.5Sb_1.5Te₃ powder and BMImTFSI at 573 K by hot pressing, and its microscopic structure and thermoelectric properties were investigated. The dispersed micropores, where BMImTFSI occupied, were observed by SEM observation on the cross-section of Bi_0.5Sb_1.5Te₃-BMImTFSI sintered composite. With an increase of the amount of BMImTFSI, the Seebeck coefficient of the composite generally decreased, while the electrical resistivity increased and the electrical resistivity of the composite with 0.5 ml of BMImTFSI is about two times larger than that of Bi_0.5Sb_1.5Te₃ with no BMImTFSI. The thermal conductivity of the composite with 0.01 ml of BMImTFSI was reduced by about 20 % compared to that of Bi_0.5Sb_1.5Te₃ with no BMImTFSI. These results indicate that the dispersed BMImTFSI in the composite acts as a scattering center for electric carrier and phonon. As a result, the dimensionless figure of merit ZT of the composite with 0.01 ml of BMImTFSI showed a maximum value of 0.70 at 373 K, which was about 20 % larger than that of Bi_0.5Sb_1.5Te₃ with no BMImTFSI.