Nanofibers from poly(lactic acid) (PLA) homopolymer and poly(p-dioxanone)-b-poly(ethylene glycol)  multi-block copolymer (PPDO-b-PEG) with different phase separation morphologies depending on the crystallization induced self-assembly of PPDO-b-PEG are prepared by single spinneret electrospinning. Mixing solvents of chloroform/dimethyl formamide (CHCl₃/DMF) with different composition are used for controlling the crystallization of PPDO block and therefore the phase separation in obtained nanofibers. The crystallization of PPDO block has very important influence on the morphology of the PPDO-b-PEG nanoparticles in spinning solution. In spinning solution with low DMF content, nanoparticles with irregular shape and incompact inner structure are formed because the degree of crystallization of PPDO block is relatively low, and a discontinuous sea-island phase separation is formed in the obtained electrospun nanofibers. Meanwhile, in spinning solution with high DMF content, the copolymer can form flake-like nanoparticles with relatively high degree of crystallization. The flake-like shape is in favor of compact aggregation of PPDO phase during formation of the nanofibers, and a continuous core-shell phase separation of the nanofibers is obtained.