[en] We have synthesized the half-Heusler ZrNiSn alloys where the Ni is partially substituted by Co and/or Cu. Such substitution much affects the thermoelectric properties of ZrNiSn by changing the valence electron count (VEC) of the material. The behaviors in the Seebeck coefficient and electrical resistivity for ZrNi_1-xCo_xSn or ZrNi_1-yCu_ySn system can be understood as the result of the increase in the hole or electron carrier with an increase of x or y, respectively. ZrNi_0.98Cu_0.02Sn, whose VEC is 18.02, showed a maximum power factor. We have also synthesized the samples whose Ni site of ZrNiSn is substituted by both Co and Cu atoms, ZrNi_1-x-yCo_xCu_ySn. In order to maintain the optimum VEC, the value of y-x is fixed at 0.02. The Seebeck coefficient of ZrNi_1-x-yCo_xCu_ySn is almost constant independent of x and y. With increase of x and y, the thermal conductivity decreases, while the electrical resistivity increases. The decrease of the thermal conductivity and the increase of the electrical resistivity are ascribed to the enhancement of the scattering of the phonon and the conduction carrier accompanied with the substitution, respectively. As a result, ZrNi_0.98Cu_0.02Sn shows a maximum dimensionless figure of merit of about 0.22 at 570 K, which is some larger than that of ZrNiSn