Cu nanowires with diameter of about 50 nm were electrochemically deposited in anodic alumina membrane. An in-situ high-temperature XRD technique was used to measure the crystalline structures. The results demonstrate that these Cu nanowires have a thermal expansion coefficient of about 2.6 × 10-5 K-1, which is a little larger than the bulk's one. These polycrystalline Cu nanowires are completely different from the zero-expansion Cu nanowires reported in previous reference. It is concluded that the thermal expansion coefficient is dependent on the crystalline perfection in nanowires.