We present semi-polar GaN light-emitting diode (LED) structures grown on non-patterned Si(112) and Si(113) substrates by metal organic vapor phase epitaxy. Cathodoluminescence and field emission scanning electron microscopy are used for sample characterization. A correlation between the structural and optical properties of the semi-polar GaN LED structures is observed. In samples, which were simultaneously grown on Si(112) and Si(113) under growth conditions optimized for Si(112), we observed that structures on Si(112) consist of a relatively smooth surface but those on Si(113) have a large number of surface pits with a three dimensional growth mode of the GaN layers resulting in a rough GaN surface. The growth conditions were further optimized to obtain smooth GaN layers on Si(113) and compared to the sample on Si(112). (C) 2012 Elsevier B.V. All rights reserved.