Abstract Half-Heusler ZrNiSn alloys, where Zr or Sn site is substituted by Y or Sb, respectively, were synthesized and their thermoelectric properties were investigated. Such substitution much affects the thermoelectric properties of ZrNiSn by changing the valence electron count (VEC) of the material. The behaviors in the thermoelectric properties for Zr 1− x Y x NiSn or ZrNiSn 1− y Sb y system can be understood as the result of the increase in the hole or electron carrier with an increase of x or y , respectively. The substitution for Zr site by Y in ZrNiSn is effective for the reduction of the thermal conductivity, while the substitution for Sn site by Sb in ZrNiSn is effective for the reduction of the electrical resistivity. We have also synthesized the double-substituted alloys, Zr 1− x Y x NiSn 1− y Sb y . The conducting property of Zr 1− x Y x NiSn 1− y Sb y is mainly dominated by the content of Sb. The phonon scattering is almost affected by the total contents of the substituent regardless of the substituted site or element. As a result, ZrNiSn 0.98 Sb 0.02 shows a maximum ZT of 0.28 at 573 K in this investigated Zr 1− x Y x NiSn 1− y Sb y system.