Electrospun polyacrylonitrile (PAN) fiber precursor-based carbon nanofiber (CNF) mats were produced and impregnated with epoxy resin. The mechanical properties of as-prepared nanofibers in the mat and short fiber filled epoxy nanocomposite forms were determined to demonstrate the effect of fiber aspect ratio on those properties. The experimental results reveal that epoxy nanocomposites containing electrospun carbon nanofibers (ECNF) with high fiber aspect ratio in the non-woven mat form yield better mechanical properties than those filled with short ECNFs. The ECNF mat in epoxy nanocomposites provides better homogeneity and easier preparation than short ECNFs. Mechanical properties of ECNF mat-epoxy nanocomposites, which were obtained using tensile and flexural tests, such as stiffness, increased, while toughness and flexural strength decreased, compared with the neat epoxy resin. Dynamic mechanical analysis results showed higher modulus for ECNF mat-epoxy nanocomposites, compared with those for neat epoxy resin and short ECNF-epoxy nanocomposites. The ECNF-epoxy nanocomposites had higher storage and Young's moduli with 1.23, 3.56 and 9.28 wt% ECNF mat loadings for the storage modulus and 0.98, 2.06% ECNF mat loadings for Young's modulus, even though the glass transition temperature (Tg), values dropped at all these extents of ECNF mat contents when compared with the neat epoxy resin.