We demonstrate that thin AlN buffer layers improve the orientation of GaN nanowire grown on Si(111). The deposited GaN initially forms into islands which act as a seed for the wires. By raising substrate temperature, the actual amount of grown material decreases but wire density increases and well-separated wires are achieved. Fast wire length growth rate at high growth temperature is assigned to an enhancement of adatom diffusion. The upper limit of length growth rate is determined by the supply rate of active nitrogen.