The growth of one dimensional Al-doped ZnO nanowires on a silicon substrate with a ZnO seeding layer by using a low temperature hydrothermal method is demonstrated. The structural and optical characteristics of the solution-grown Al-doped ZnO nanowires were studied using field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), and cathodoluminescence (CL) measurements. The FE-SEM images show that the ZnO nanowires are hexagonal column shaped and stand perpendicularly on the silicon substrate. It is found in XRD patterns that the ZnO nanowires annealed at various temperatures are all wurtzite crystal structure and have (002) preferred orientation. As the annealing temperature changed, the intensity of (100) and (101) peaks reveals distinct change. The CL spectra show the as-grown ZnO nanowires display a weak and narrow UV emission centered at 376 nm. A strong and very broad green-yellow emission band with a center at 575 nm as well as a shoulder like orange-red emission band centering at 625 nm is also demonstrated. It is shown that the luminescence intensity centered at 376 nm increased, the shoulder like emission centered at 625 nm decreased but emission centered at 575 nm disappeared after thermal annealing treatment in oxygen atmosphere.