Abstract: Epoxy resin was modified with liquid nitrile-butadiene rubber (LNBR), and unidirectional boron fiber/epoxy resin (B_f/epoxy) composites were fabricated with compression molding. Mechanical properties of the composites without and with 10% LNBR were tested. Failure modes of the unidirectional B_f/epoxy composites without and with 10% LNBR under tensile loads were studied by observing their macro- and micro-fracture surfaces. The results show that 10% LNBR increases the tensile strength, bending strength, interlaminar shear strength and elongation to fracture of the unidirectional B_f/epoxy composite by 18.42%, 13.39%, 28.45% and 43.40% , respectively, while there is a decrease of the tensile modulus and bending modulus. The failure mode is of a mixed fracture mode between cohesion failure and debonding failure on the fracture surface of the unidirectional B_f /epoxy composite with 10% LNBR.