By using a double cantilever beam (DCB) specimen, the hygrothermal effect on Mode II (in-plane shear loading) interlaminar fracture toughness of a carbon/polyphenylene sulfide (PPS) unidirectional laminate was investigated. The critical strain energy release rates, G nc , of the carbon/PPS laminate during crack propagation under Mode II loading for various temperature/relative humidity conditions were obtained both by linear beam theory and by the area method. It was found that the critical strain energy release rate of the carbon/PPS laminate increases with both moisture content and temperature. Fracture of the carbon/PPS laminate was characterized primarily by micro-cracking at low temperature/low moisture content and by matrix softening and fiber bridging at high temperature/high moisture content.