In an effort to predict the long term durability of carbon fiber/epoxy composites in a space environment, interlaminar shear strength (ILSS) of the composites was measured as a function of 0.5 MeV electron radiation dosage. For the ILSS measurements, a notch method (ASTM D3846) was used with and without side-supports. The supports were used to prevent peeling or bending during the test. The ILSS of both T300/5208 and T300/5209 longitudinal composite system increases monotonically with radiation when the test is done with the support. When the test is carried out without the support, the ILSS of the composites increases initially but then decreases with further radiation. It is also observed that the ILSS of the unsupported case is much lower than that of the supported case. Measurement of epoxy modulus shows that the elastic modulus increases monotonically with radiation. But the breaking strength of the epoxy decreases with radiation. Electron Spectroscopy for Chemical Analysis shows that the oxygen contents at both the pure epoxy surface and the composite fracture surface increase with radiation dose, resulting in the increase of polarity at the interfacial region. This may be a supporting evidence for the increase in the ILSS of the composites.