Abstract: Hygrothermal behavior of unidirectional carbon fiber reinforced BMI matrix composite laminates was investigated，which were subjected to three-point flexural load in the distilled water at 70℃ for different times. Compared with the unstressed specimens，there are no significant differences in apparent moisture absorption，as the glass transition temperature and residual flexural properties were observed for flexural-stressed specimens. However，the flexural-stressed specimens show a greater decrease in glass transition temperature before saturation. The tensile-stressed section absorbs water faster than the compressive-stressed section for the flexural -stressed specimens. The SEM of the flexural fracture surface shows that the adhesion property of the fiber and matrix in the tensile-fracture section of the stressed specimens suffers more severe deterioration than that of the unstressed specimens. It indicates that the stress has significant influence on the hygrothermal property of the composite，which depends on the direction of the stress.