The thermal expansion behavior of semiconductor single-crystal nanowire arrays is of importance for their applications in electronic and optoelectronic nanodevices. We prepare hexagonal ZnS single-crystal nanowire arrays growing along the  direction via electrodeposition. The thermal expansion properties of the as-prepared ZnS nanowires have been studied by in situ x-ray diffraction method. The thermal expansion coefficient (TEC) of the ZnS nanowires decreases consistently from room temperature to 225 degrees C where it reaches a minimum value, and then increases rapidly. The average TEC in the studied temperature range is 4.74 x 10 (6)/degrees C, which is smaller than that of the conventional bulk counterpart.