Through single-tube electrospinning, the biodegradable core-shell fibers of poly(3-hydroxybutyrate) (PHB) and poly(D,L-lactic acid) (PDLLA) were obtained from blend solutions with different compositions at a total polymer concentration of 7 wt %. Regardless whether PHB is the major or minor component (PHB/PDLLA = 90/10, 75/25, 50/50, and 25/75 wt. ratio), these phase-separated solutions all yielded core-shell fibers with PHB as core and PDLLA as shell. A new scenario of core-shell fiber formation was proposed on the basis of the relative magnitude of the intrinsic relaxation rate of fluids and external extension rate during electrospinning. The effects of blend compositions on the morphologies of the Taylor cone, whipping jet, and as-spun fibers were investigated. The diameters of core-shell fibers can be tailored by simply varying the PHB/PDLLA ratios. Two scaling laws describing the apparent-viscosity (eta(o)) dependence of the outer fiber diameter (d(fo)) and core fiber diameter (d(fc)) were derived. That is, d(fo) similar to eta(0.38)(o) and d(fc) similar to eta(0.86)(o). The microstructures of the as-spun fibers were determined by differential scanning calorimetry, Fourier transform infrared spectroscopy, and synchrotron wide-angle and small-angle X-ray scatterings. Results showed that the PDLLA component was in the amorphous state, and the crystallizability of PHB component remained unchanged, except the amorphous 10/90 fibers electrospun from a miscible solution state. The synergistic mechanical properties of the core-shell fibers were obtained, along with the ductile PDLLA shell endosing the brittle PHB core. The enhanced toughness was attributed to the fragmentation of the brittle PHB core and necking fracture of the ductile PDLLA shell, which served as an effective route for energy dissipation. Compared with the neat PHB fiber, the 90/10 and 75/25 core-shell fibers possessed larger elastic moduli, which was attributed to the high PHB crystal orientation in their core sections despite the reduced PHB crystallinity. By contrast, the crystal c-axis of PHB in the 25/75 core-shell fibers was preferentially perpendicular to the fiber axis, suggesting the significant stretching of developing PHB crystals during electrospinning.