We demonstrate in this work the effects of flow modulation on the crystalline quality and morphology of GaN epilayers grown by hydride vapor phase epitaxy. The improvement in the structural qualtiy is attributed to the strain relaxation, dislocations suppression and enhanced Ga diffusion.Introduction In the past years GaN has been extensively investigated due to its potential in the fabrication of optoelectronic devices operating in the ultraviolet and violet spectral regions. However, conventional growth techniques usually have difficulty in growing GaN layers with good morphology as well as low dislocation density. Up to date, additional techniques such as pretreatment of substrates [1-3], predeposition of a buffer layer [4,5], and epitaxial lateral overgrowth (ELOG) have been utilised to improve the crystal quality of GaN films [6-10]. For example, Nakamura et al.[7] demonstrated a significant increase in the lifetime of blue laser diodes by employing the ELOG technique for the growth of underlying GaN layers. Recently, pendeo-epitaxy [8] and lateral overgrowth from trenches have also been developed to reduce the dislocation densities in GaN layers. Lateral epitaxial overgrowth with NH 3 flow rate modulation was recently introduced to improve the morphology and lateral growth of GaN by metalorganic vapor phase epitaxy [9, 10]. As an alternative, in this study we develop flow modulation growth to improve the GaN films grown by hydride vapor phase epitaxy (HVPE). Our motivation was the following: a) since the dislocations result from the lattice mismatch between substrate and film and then thread into upper layers during the growth process, we hope to interrupt the threading dilocations and suppress them into the initial layers via growth interruption; b) the strain in the sublayers may have time to relax due to the growth interruption and has less influence on subsequent layers; and c) the variation of III/V ratio during the modulation process may be helpful to modulate the GaN growth mode [9,10]. It is demonstrated that high quality GaN films can be grown by hydride vapor phase epitaxy via flow modulation of the NH 3 and/or Ga sources.