In this paper, we describe the effects of cryogenic temperature at 77 K and water absorption on the mechanical properties and fatigue strength of intermediate-temperature cured carbon fiber reinforced epoxy matrix composites. Tensile and tension-tension fatigue tests were conducted on both dry and wet specimens, which had been preconditioned in laboratory air and in water for two months, respectively. The test results showed that i) an increase in the strength of the fiber/matrix interface and matrix material and in the elastic modulus of the matrix at cryogenic temperature, ii) a decrease in interlaminar strength at cryogenic temperature, and iii) a decrease in fiber/matrix interfacial and interlaminar strength by water absorption severely influenced the mechanical and fatigue fracture behavior of the composites. Internal damage was non-destructively examined with a scanning acoustic microscope, the fracture surfaces were closely examined with a scanning electron microscope, and the influence of cryogenic temperature and water absorption on fracture mechanisms was discussed.