Single fiber breaking force (in gram force) and tenacity (in g/tex) of developing SJ-2 cotton fibers have been studied. Wide angle X-ray diffraction (WAXD) techniques provide crystalline structural information of dried cotton fibers at the same development stages. The increased crystallinity and crystallite sizes and perfection offer only partial explanation to the strength of cotton fibers. Other structural factors including orientation of microfibrils, inter-crystallite and inter-fibril hydrogen bonding, and inter-crystallite and inter-fibril stress can also contribute to single fiber strength.