This study was designed to investigate the crystallite sizes of Porites species. A set of 9 Porites skeletons i.e. Porites australiensis, Porites cylindrica, Porites lutea, Porites lichen, Porites digitata, Porites nigrescens, Porites rus, Porites annae and Porites sp. were studied by using XRay Diffraction method. The values of FWHM and theta (θ) are used in Scherrer equation to determine the crystallite sizes of all Porites samples. It was found that the crystallite sizes were in the range of 1007.78 Ǻ – 1706.04 Ǻ.