NOVELTY - A composite thermoelectric material comprises mother phase including thermoelectric material, half-Heusler compound and aggregate of crystal grain(s), and oxide layer containing metal oxide having band gap more than the mother phase. The oxide layer covers surrounding of the mother phase in continuous and layered form. The composite thermoelectric material has interface coverage of 50% or more, which is calculated by specific formula (I). The total length of mother phases has aspect-ratio of 10 or more and thickness of 1-500 nm.    USE - Composite thermoelectric material e.g. powder and sintered compact (claimed). Uses include but are not limited to solar power generation device, fossil fuel thermoelectric generator, optical detection element, laser diode, field effect transistor, photomultiplier tube, cell of spectrophotometer, chromatography column, thermostat, refrigerator, timepieces, etc.    ADVANTAGE - The composite thermoelectric material has increased Seeback coefficient without reducing electrical conductivity and reduced thermal conductivity. The conduction of low-energy carrier or decimal carrier and photon conduction are inhibited.    DETAILED DESCRIPTION - Composite thermoelectric material comprises mother phase including thermoelectric material, half-Heusler compound ((M1-aAa)1+x(Ni1-bB'b)1+y(Sn1-cC'c), where a, b and c are 0 to less than 0.1, x is (-0.1)-0.2, y is 0-0.2, M is chosen from titanium, zirconium and hafnium, A is chosen from IIIa group element, IVa group element (excluding titanium, zirconium and hafnium), Va group element, and rare earth elements, B' is chosen from VIIIa group element (excluding nickel), and Ib group element, C' is chosen from IIIb group element, IVb group element (excluding tin), and Vb group element) and aggregate of crystal grain(s), and oxide layer containing metal oxide having band gap more than the mother phase. The composite thermoelectric material has interface coverage of 50% or more, which is calculated by specific formula: Interface coverage=( sigma )(Lix 100/L0i)/n (I), where L0i is interface length of ith mother phase, i is 1-n, n is 5 or more and Li is interface length of a portion at which oxide layer covers surroundings of the ith mother phase, where aspect-ratio is 10 or more and thickness is 1-500 nm. An INDEPENDENT CLAIM is included for manufacture of composite thermoelectric material.