Abstract: The hygrothermal stresses in the composite with fiber volume fraction of 30% were investigated by using micro Raman spectroscopy. The results show that the Raman wavenumber shift for carbon fibers can be caused by the hygrothermal stresses，which can be used to characterize the hygrothermal stress in the fibers. The appropriate selections of the tested points are the key to success in the hygrothermal stress test with Raman spect roscopy for the composites. The results also show that during the long-term moisture absorption process，the axial residual stresses within the fibers transform from thermal residual compressive stresses before absorption into hygrothermal tensile stresses after absorption. The axial stresses within the fibers induced purely by the absorption can be obtained by using the tensile stresses after absorption minus the compressive stresses before absorption，which is rather high (2272 MPa on average) and close to the tensile strength of 2800 MPa of the carbon fibers used in this paper. The suitable thermal residual stresses can reduce the moisture induced stresses to some extent.