Multiscale glass fiber epoxy matrix composites containing nanodiamonds were fabricated using vacuum bagging technique. Three different loadings of nanodiamonds were incorporated in epoxy resin after their functionalization through ozone-treatment, i.e., 0.1, 0.3 and 0.5 wt%. The functionalization of nanodiamonds was confirmed by infrared spectroscopy, which improved the dispersion of nanodiamond in epoxy resin thus improving the mechanical properties. Tensile, compression, flexural and interlaminar shear properties of the composites were improved. The tensile, compression and flexural strengths improved up to 36, 56 and 30% by the addition of 0.5 wt% nanodiamonds while the corresponding moduli increased to 30, 125 and 46%, respectively. An improvement of 38% in interlaminar shear strength was observed. The microscopy of the composites was performed using optical and electron microscopy and proper impregnation of glass fibers and the absence of the agglomerates of nanodiamonds were ensured. The homogeneous dispersion of nanodiamonds and their adhering role at fiber/matrix interface improved the mechanical properties of the composites. The developed composites are ideal candidate materials for engineering applications demanding high specific mechanical properties. [GRAPHICS] .