The effect of water environment on delamination fatigue crack propagation behavior was investigated using unidirectional CFRP laminates made from Toray prepregs T800H/3900-2. T800H/3900-2 has toughened interlayer which is composed of epoxy resin and thermoplastic polyamide particles. Tests were carried out under mode II loading using end notched flexure specimens which were moisture-conditioned up to the saturated level. In the static tests, the value of the fracture toughness for the moisture-saturated specimen was slightly smaller than that for the dry specimen in air. In the fatigue tests, the crack propagation rate, da/dN, was expressed as a power-low function of the maximum energy release rate, G_{II max} in the region where da/dN>5×10^-10m/cycle. Below this region, there exists the threshold value of G_{II max}. The threshold value for the moisture-saturated specimen in water was smaller than that for the dry specimen in air. Glass transition temperatures were measured both in moisture-saturated state and in dry state using differential scanning calorimetry. The decrease in the glass transition temperature indicated the ductility increase of epoxy. Microscopic observation revealed that the ductility increase of the epoxy affected the morphology of the fracture surfaces under fatigue loading.