We study theoretically and experimentally the time evolution of GaAs nanoneedles grown by metal organic chemical vapor deposition on silicon and sapphire substrates. A theoretical model is presented which provides explicitly the nanoneedle length as a function of growth time under the assumption of a fixed aspect ratio. Experimental data confirms that the aspect ratio remains approximately constant during the growth. From fitting the experimental length-time dependences, we estimate the effective arrival rates and the Ga diffusion lengths as being 3.5 nm/min and 80 nm on the silicon and 2.9 nm/min and 400 nm on the sapphire substrates, respectively. © 2011 American Institute of Physics.