Hexagonal silver nanowire (4H-Ag NW) arrays with preferred orientation are fabricated into the holes of porous anodic alumina membranes (AAMs) by direct current electrodeposition. The addition of tartaric acid and low deposition temperature are crucial conditions for the growth of 4H-Ag NWs. The thermal expansion properties of the as-prepared and annealed 4H-Ag NWs are studied by in situ x-ray diffraction in the temperature range from room temperature to 800 °C. The axial thermal expansion coefficient (TEC) of the as-prepared sample decreases initially and increases subsequently, while that of the annealed sample consistently increases during the whole measuring process.