We grew sapphire crystal fibers doped with various Mg concentrations from 0.5 to 4 mol % by the laser-heated pedestal growth method. It was easy to grow defect-free 0.5 mol % Mg-doped sapphire crystal fibers longer than 50 mm; however, the length of the defect-free region decreased as the amount of MgO doping increased. The growth rates also influenced the quality of the fibers. Microscopic observations of cross sections of the grown fibers revealed that the defects were always concentrated in the central regions of the fibers. Using an electron probe X-ray microanalyzer, we also found Mg in the central regions of the fibers. We discuss the influence of thermocapillary convection and the growth rate on the Mg distribution, and the reason for the formation of defects by constitutional supercooling is described. Using thermomechanical analyzer measurements, we suggest that the strength of the sapphire can be enhanced by MgO doping, which increases Young’s modulus.