Nanodiamonds (NDs) were employed for the first time to enhance the mechanical properties of poly(lactic acid) (PLA)-based nanofiber scaffolds. Uniform ND/PLA composite nanofibers can be electrospun at <1 wt% loading of NDs. The introduction of NDs improved the thermal stability of PLA-based nanofibers. Fourier transform infrared spectroscopy results demonstrated good adhesion between ND nanofillers and PLA matrix. Following the addition of NDs, the four mechanical indicators, tensile strength, Young's modulus, elongation at break and fracture toughness of ND/PLA composite nanofiber membranes increased accompanied by the later decrease with the rise of ND content. The four indicators achieved their respective maximum value at 1 wt% ND content, which revealed 2.4 fold increase of tensile strength, 1.6 fold augment of Young's modulus, 1.4 fold elevation of elongation at break, and 4.8 fold growth of fracture toughness, respectively. Compared with the tensile strength and Young's modulus of neat PLA, ND nanofillers exhibited the best reinforcing ability for PLA-based composite nanofibers, which was attributed to the effective interfacial adhesion between PLA and rigid ND particles and the good dispersion of NDs in PLA matrix. The ND/PLA composite nanofiber membranes with improved mechanical properties possess potential application in biomedical engineering.