Highly ordered single-crystal iron nanowire arrays with different diameters have been fabricated in anodic aluminum oxide (AAO) templates by DC electrodeposition method. The Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM) show that the iron nanowires are highly uniform and exhibit a single crystal structure. The X-ray diffraction (XRD) patterns of iron nanowire arrays indicate that most of the iron nanowire arrays have the obvious preferred orientation along the  direction. From the hysteresis loops of the iron nanowires, it reveals that the easy magnetization axes of nanowire arrays are along the long axis. The sample with smaller diameter (d=35nm) has a high square ratio (up to 98%) and a high coercive filed (1265Oe) when the external magnetic field is applied along axis of the nanowires. When the diameter decreases, the square ratio and the coercive field increase due to the single-domain structure and the strong shape anisotropy in the smaller diameter nanowire arrays.