The n-type half-Heusler alloy ZrNiSn has been well studied because of its high power factor but its high thermal conductivity leads to low figure-of-merit. Hence in the present work an attempt has been made to reduce the total thermal conductivity by a combination of substitutions. Four different ZrNiSn based half-Heusler alloys ZrNiSn0.95Ge0.05, ZrNiSn0.9Ge0.1, Zr0.75Ti0.25NiSn0.97Si0.03 and Zr0.75Ti0.25NiSn0.97Si0.02Sb0.01 have been synthesized by vacuum arc melting of pure elements. X-ray diffraction together with Rietveld refinement shows that all the alloys are essentially single phase which has a F4m cubic structure. Scanning electron microscopy shows large grains and a uniform chemical composition with very little loss of elements. Electrical resistivity shows a weak temperature dependence indicating a degenerate semiconducting behavior, 0.5 mΩcm to 6.4 mΩcm. The carrier concentration is found to be ~ 1020 cm-3. The Seebeck coefficient increases with increasing temperature and shows a bipolar behavior for T > 700 K. The ZrNiSn0.95Ge0.05 alloy has the highest Seebeck coefficient of 148 µVK-1 and a power factor of 2.9 mWm-1K-2. The thermal conductivity has a very low temperature dependence indicating significant electronic contribution. The ZrNiSn0.95Ge0.05 alloy has a thermal conductivity of ~ 6 Wm-1K-1 while the Zr0.75Ti0.25NiSn0.97Si0.03 alloy has the lowest thermal conductivity of ~ 3.2 Wm-1K-1. The ZrNiSn0.95Ge0.05 alloy exhibits the highest figure-of-merit of 0.45 at 1023 K.