The improvement of crack resistance is essential to the application of fiber reinforced composites for cryogenic use. In this study, the authors attempted to enhance the crack resistance of a carbon/epoxy composite by adding multi-walled carbon nanotubes (MWNTs) into the resin formulation. Prior to assessing the effect of MWNTs, the effect of a toughened epoxy resin on the mode I interlaminar fracture toughness was investigated at -150 °C using double cantilever beam (DCB) specimens. It was found that the degree of fracture toughness enhancement obtained from application of the toughened epoxy at cryogenic temperature was less than that at room temperature, due to embrittlement of the epoxy resin. MWNT-added carbon/epoxy unidirectional prepregs were fabricated via a filament winding method with different concentrations of MWNTs (0.0 wt% for baseline, 0.2 wt% and 0.7 wt%). Material systems blended with 0.2 wt% and 0.7 wt% of MWNTs showed enhanced fracture toughness and low crack density at the cryogenic temperature.