We demonstrate a 400 mu m-thick GaN layer on 4 inch (0001) Al2O3 substrates through GaN nanodot formation as a seed for stress relaxation layers, which were formed by an in situ special surface treatment using HVPE. The size and density of the GaN nanodots determined the thickness of the stress relaxation layers and the structural properties of thick GaN. The 400 mu m-thick GaN layer exhibits a smooth surface and high crystal quality with FWHM of 104 arcsec and 163 arcsec in the (002) and (102) X-ray rocking curves, respectively. The dislocation density estimated via micro-PL measurements was 2 x 10(6) cm(-2). This can provide an efficient and simple way to fabricate thick GaN layers on an Al2O3 substrate without ex situ buffer layer formation or additional complicated processes.