This paper investigates ferroelectric domain structures in MgO:LiNbO3 single-crystal fibers grown by LHPG. The origins of the striation domain structures (SDS) are attributed to thermocapillary instability due to MgO doping. It is more difficult to form SDS in the core regions of the fibers because of the poor impurity content. The SDS on the (01.0) face, along the [2 (1) over bar.0] direction, are mixed with the hi-domain and are weaker compared with those observed along the [00.1] direction. MgO dopants, pulling rates and fiber diameters are all regarded as important factors towards the configuration of the SDS. (C) 1999 Elsevier Science B.V. All rights reserved.