A series of GaN thin films were grown on Si substrate under different conditions using metalorganic chemical vapor deposition (MOCVD) and characterized by Nomarski microscopy (NM), optical reflectance (OR), high-resolution X-ray diffraction (HRXRD), Raman scattering (RS) and photoluminescence (PL). NM showed different patterns for GaN/Si grown under different growth and source flow parameters. XRD, RS and PL measurements confirmed their wurtzite crystalline GaN structures, and the corresponding line shape analyses revealed their difference corresponding to the NM observations. The control and improvement of the crystalline cracking in GaN/Si are discussed. (c) 2005 Elsevier B.V. All rights reserved.