The development of internal microstructure in electrospun fibers has been investigated both experimentally and theoretically. Various morphologies such as tubes, beads, and porous structures have been observed experimentally during electrospinning from amorphous polymer solutions such as poly(methyl methacrylate)/methylene chloride and poly(styrene)/tetrahydrofuran. The dynamics of electrospinning is modeled based on multiple virtual strands of beads connected by Maxwell's elements in a cylindrical coordinate system. Concurrently, spatiotemporal growth of the porous structure is calculated in the framework of Cahn-Hilliard time-evolution equation under the quasi steady state assumption coupled with the solvent evaporation rate equation. The coarse-grain simulation reveals the real-time formation of pores along the spinline of the electrospun fiber as the concentration traverses across the phase diagram of the amorphous polymer solution.