GaN films were grown by metalorganic chemical vapor deposition (MOCVD) on silicon nitride (SiN)-treated c-plane sapphire substrate. Using in situ laser reflectometry, the growth was interrupted at the different stages of the film coalescence. By the help of high-resolution X-ray diffraction (HRXRD) and Hall effect measurements, the mosaicity, stress and electrical properties were investigated at all the growth stages. The results showed that tilt and twist mosaic drop with thickness increasing at different rates. During the coalescence process, interaction between tilt and twist mosaic increases and remain almost constant for fully coalesced layers. Hall effect measurements have shown that carrier concentration and mobility have the same sensitivity to the tilt and the twist mosaic. The a and c lattice parameters, as well as the in plane and out-of plane strains were determined. The room temperature compressive stress increases with increasing layer thickness and reaches a maximum level of -0.45 GPa, The effects of SiN treatment on the mosaicity and the stress evolution were discussed from a comparison with the properties of untreated GaN grown layer. (C) 2007 Elsevier B.V. All rights reserved.