In the fabrication of silicon optical fibers, there is a need to understand the dependency of the grain size of the silicon core and the processing parameters during the fabrication process. This study was undertaken to determine how the grain size in the core structure varied with changes in the fabrication parameters during drawing of the silicon optical fibers. The primary fabrication variables' changes were preform dimension and drawing speed, resulting in fibers of varying diameters in both the core and cladding regions. Grain size and orientation of the silicon core of the resultant fiber was determined using Electron Backscatter Diffraction (EBSD). Grain size was normalized to the core diameter and it was found that the grain size or the length of the grain within the core increased as the diameter of the core increased. No preferential grain orientation was observed in any of the fibers. Grain length increases as the diameter increases and the rate of cooling decreases as the diameter increases. This decrease in cooling rate reduces the nucleation rate of grains during solidification process as the fibers are being drawn. (C) 2013 Elsevier B.V. All rights reserved.