Abstract: By simulating geosynchronous orbit space γ ray irradiation environment, the high modulus carbon fiber and carbon fiber/cyanate ester composites were irradiated by ⁶⁰Co-γ ray. The carbon fiber and carbon fiber/cyanate ester composites were analyzed and characterized by SEM and XRD before and after irradiation. The effects of irradiation on the mass loss rate, tensile strength and interlaminar shear strength were studied. The results show that the mass loss rates increase with the increase of γ ray dose and then tend to be gentle, but less than 1%; the tensile strength and interlaminar shear strength of carbon fiber/cyanate ester composites both increase firstly and then decrease with increasing dose, the maximum values occur at 5×10⁵ rad with tensile strength of 1 803 MPa, tensile modulus of 243 GPa and interlaminar shear strength of 72 MPa.