The effects of surface treatments, such as oxygen plasma and nitric acid treatment, on the mechanical properties of carbon fiber (CF)/polybenzoxazine composites were investigated. Interlaminar shear strength (ILSS) and flexural strength of CF/polybenzoxazine composites were measured and correlated with surface area and surface functionality. Both oxygen plasma treatment and nitric acid treatment were efficient in the enhancements of the ILSS and flexural strength of CF/polybenzoxazine composites. However, nitric acid treatment was more efficient than oxygen plasma treatment due to large increment of surface roughness. Cohesive failure was observed in surface treated composite due to adhesion improvement.