The microstructure and growth behavior for vertically aligned Zinc oxide (ZnO) nanowires, synthesized on a ZnO thin film template by pulsed-laser deposition (PLD), is reported. The nanowire growth proceeds without any metal catalyst for nucleation, although an epitaxial ZnO thin film template is necessary in order to achieve uniform alignment. Nanowire growth at argon or oxygen background pressures of 500-mTorr results in nanowire diameters as small as 50-90 nm, with diameters largely determined by growth pressure and temperature. Room temperature photoluminescence show both near-band-edge and deep-level emission. The deep-level emission is believed caused by oxygen vancancies formed during growth.