An electrospinning process was successfully used to fabricate polyethylene oxide/cellulose nanocrystal (PEO/CNC) composite nanofibrous mats. Transition of homogeneous to heterogeneous microstructures was achieved by tailoring the concentration of PEO/CNC mixture in the solution from 5 to 7 wt %. Morphology investigation of the obtained nanofibers demonstrated that rod-shaped CNCs were well-dispersed in the as-spun nanofibers and highly aligned along the nanofiber long-axis. PEO/CNC nanofibers became more uniform and smaller in diameter with increased CNC-loading level. The heterogeneous composite mats were composed of rigid-flexible bimodal nanofibers. Results of structure characterization indicated that the incorporated CNCs interacted strongly with the PEO matrix through hydrogen bonding. Mechanical properties of both types of mats were effectively improved by using CNCs, with heterogeneous mats being stronger than their homogeneous counterparts for all compositions (0-20 wt % CNC contents). When a smaller diameter needle was used to form homogeneous mats, enhanced thermal and mechanical properties were obtained.