Melt electrospinning is an effective processing technique to fabricate microfibers for many applications such as tissue scaffold engineering, catalysts, fabrics, and filters. Compared to solution electrospinning, melt electrospinning does not involve a solvent and thus it is beneficial for biomedical applications such as the tissue engineering of cell constructs where solvent toxicity or accumulation is a major concern. Two limitations of melt electrospinning are: (i) a larger fiber size (approximately 100µm) compared to that of solution electrospinning, and (ii) unsatisfactory fiber deposition control which prevents creating a well-defined 3D geometry. In this study, we developed a novel fiber processing technology which combines fused deposition modeling (FDM) based 3D printing and melt electrospinning using Polycaprolactone (PCL). The objectives of this study are two folds; (i) to investigate and optimize the significant parameters associated with the melt electrospinning process that influence fiber diameter, including applied voltages, distances between nozzle and counter electrode, and processing temperatures in order to produce microfibers with less than 100 um diameters, and (ii) to develop 3D structures with two different microfiber morphologies which consist of electrospun fiber mats surrounded by well-defined 3D boundary structures by FDM 3D printing. Scanning electron microscopy (SEM) was used to image the 3D structures with bi-modal microfiber distributions and the fiber diameters were measured using Quartz-PCI Image Management Systems in SEM.