Formation of various domain shapes in submicron scale fibers of poly(styrene-block-isoprene) (PS-b-PI) has been investigated via electrospinning. Monodisperse PS-b-PI block copolymers with 29 and 53 vol % of PI were synthesized using two-step anionic polymerization and were dissolved in tetrahydrofuran (THF). Solutions of block copolymer with varying concentrations in THF were electrospun, and fibers with average diameters from 200 nm to 5 mu m were obtained. Small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) studies revealed that cylindrical and lamellar morphology can be formed in electrospun fibers of 29% and 53% PI copolymers, respectively. We note that these domain structures in fibers are not as well developed as those in films possibly due to the short residence time and strong elongational deformation involved in the electrospinning process. For both systems we find that the d spacing in electrospun fibers is smaller than that in the cast film. This could also be attributed to the elongational deformation and fast solvent evaporation during electrospinning. The domain structures of electrospun fibers from the symmetric (53% PI) copolymer exhibit the influence of fiber morphology such as confinement and curvature due to its high molecular weight. More uniform domain structures in the fibers and increase in d spacing are observed after the annealing process.