In this work, we illustrate and analyse long propagating nanopipes in thick HVPE-GaN layers studied by means of transmission electron microscopy. The nanopipes appear to have the characteristics and behaviour of screw-dislocations. They propagate with a constant diameter along tens of micrometers and a pit of a shape of inverted pyramid forms when the top surface of the layer is reached. The generation of the nanopipes is discussed in terms of their relation to the growth mechanism of HVPE-GaN material and the kinetics of screw dislocations in the early stages of growth of highly strained material. The nanopipes appear to promote the crystal growth when the substrate surface does not provide a sufficient nucleation base for subsequent HVPE growth. In this case they mediate the surface morphology terminating surface steps by hexagonal pits and act as centres of growth spirals. When the substrate surface provides a sufficient amount of nucleation sites the step flow mechanism is prevailing and no growth spirals associated with nanopipes are observed. (C) 2003 Elsevier Science B.V. All rights reserved.