Cu2Se based thermoelectric materials are of great potential for high-temperature energy harvesting due to their high-temperature figure-of-merit (zT). For further development of Cu2Se, both engineering and mid-temperature figure-of-merit need to be improved. In this work, we report that carbon-coated boron (C/B) nanoparticles incorporation can significantly improve both mid- and high-temperature zT in Cu2Se. The nanoparticle inclusions can result in a homogeneous distribution of Cu:C:B interfaces responsible for both improvement of the Seebeck coefficient and significantly reduction in thermal conductivity. Ultrahigh mid- and high temperature thermoelectric performance with zT = 1.7 at 700 K and 2.23 at 1000 K as well as significantly improved engineering zT are achieved in the C/B incorporated Cu2Se with desirable mechanical properties and cycling stability. Our findings will stimulate further study and exploration for the Cu2Se based thermoelectric materials for broad applications in converting waste heat to electricity with competitive energy conversion efficiency.