Abstract: The factors that seriously affect the interlaminate shear properties of unidirectional T700/epoxy composites in seawater environment were studied, as well as the variation of interlaminate shear properties and water adsorption ratio of unidirectional T700/epoxy composites immersing in seawater and distilled water after five immersing periods(15, 40, 70, 165 and 400 d). The shear fracture microstructures of unidirectional T700/epoxy composites were observed by SEM to reveal the mechanism of propertiy decline. The experimental results show that the water adsorption ratio of unidirectional T700/epoxy composite increases with the extending of immersion time, and agrees well with the Fick's law on the whole. There is a significant decline in interlaminate shear properties of unidirectional T700/epoxy composite by almost 10% after immersing for 400 d, and the variation trend of interlaminate shear properties is similar to that of water adsorption ratio. The sensitivity of interlaminate shear properties of unidirectional T700/epoxy composite to seawater is greater than that to distilled water. The cross-sectional microstructure of sample shows that the degradation of the interface between resin and fiber caused by the absorbed seawater is the main reason for the decline in interlaminate shear properties. Therefore, more attention should be paid on the interlaminate shear properties while employing the unidirectional T700/epoxy composite in seawater.