Abstract: With mixed-mode flexure (MMF) specimens, the mixed-mode interlaminar fracture toughness of composite laminate was investigated experimentally, mainly focused on the hygrothermal effect on the strain energy release rates. It was found that: the co-function of heat and moisture caused plasticity near the tip of delamination. The critical energy release rates were calculated with two expressions (for mode Ⅰ and mode Ⅱ components, respectively) based on the beam theory. Scatter graphs of G_Ⅰ versus G_Ⅱ, the mode Ⅰ and mode Ⅱ components of critical energy release rates in different hygrothermal conditions, were given. The result showed that at room temperature, the hygroscopic absorption had little effect on the interlaminar fracture toughness, while at higher temperature conditions, the toughness increased as the hygroscopic degree increased. For dry laminates, the fracture toughness showed no distinct difference as the temperature increased, and for highly moisture absorpted specimens, the toughness was improved markedly. The fractograph was examined for further understanding of the mechanism.