The interrelation between atomic switching disorder and thermoelectric properties in the half-Heusler alloy ZrNiSn was investigated. ZrNiSn samples were prepared by a time-efficient levitation melting and spark plasma sintering procedure. High-resolution synchrotron radiation powder X-ray diffraction shows that a single phase half-Heusler compound has been obtained. Trace impurities were detected after annealing at 970 K for 5 days. Rietveld refinements were carried out for both unannealed and annealed ZrNiSn samples to study the possible structural disorders. It is found that the generally accepted Zr/Sn antisite defects are not likely to exist. Instead, the refinements revealed interstitial fractional occupancy of Ni on the (½, ½, ½) site, which is normally empty in the half-Heusler phases, but filled in full Heusler materials. The electrical conductivity and Seebeck coefficient from 300 to 900 K of the unannealed and annealed ZrNiSn displayed no obvious distinction, and the room temperature electrical resistivity and absolute Seebeck coefficient of the annealed ZrNiSn even decreased slightly compared to those of the unannealed one, which implies no obvious Zr/Sn disorder transition during the annealing procedure. A slight increase in the thermal conductivity was observed after a long time annealing, possibly due to reduced Ni atomic disorder.