Abstract In this research, hygrothermal aging behaviors of carbon fiber/bismaleimide (BMI) composite materials were investigated. Water diffusivity was measured through 3 wet–dry cycles for BMI resin reinforced with unidirectional carbon fiber CCF300/QY9511 composite. The changes of the diffusion coefficient and saturation level of water absorption during the 3 cycles were determined. Electron microscopy revealed that micro-cracks near the weak interface together with the de-bonding provided routes for water uptake. The interlaminar property of composite was characterized by interlaminar shear strength (ILSS). ILSS reduction of CCF300/QY9511 from hygrothermal aging could come to a plateau during the first 14 days. The different damage morphologies between dry specimens and wet specimens were characterized by electron microscopy. ILSS under different test temperatures was also studied with an Arrhenius method, and the result of the Arrhenius method confirmed that routes, such as micro-cracks and de-bonding, for water uptake were also instrumental in speeding up the drying.