Higher manganese silicide (HMS) is a low-cost and eco-friendly thermoelectric material available for recovering waste heat of 500 to 900 K. In this research, we tried to uniformly disperse the alumina nanoparticles (ANPs) in the HMS matrix to reduce the thermal conductivity and to improve the thermoelectric performance. Influence of addition of ANPs on the thermoelectric properties was investigated. It was confirmed that ANPs were uniformly dispersed in the HMS grain boundary. The lattice thermal conductivity was reduced by adding ANPs. As a result, the maximum thermoelectric performance of ZT=0.58 was achieved at about 800 K by adding 1 vol% of ANPs. The performance of ANPs-added HMS was improved about 25 %.