Ultra-high tensile modulus (700 GPa) pitch-based carbon fiber was systematically surface treated, and interfacial shear strength (IFSS) between the fiber and epoxy resin was measured by micro-droplet method; then, relations between the IFSS and several mechanical properties of the corresponding CFRP were investigated. First, the method of measuring IFSS of very thin and fragile fiber by micro-droplet technique was developed by modifying the shape of specimen. IFSS was related to fiber surface oxygen, which seems to be related to functional groups on the fiber surface. Next, 0° tensile, 0° flexural and 0° compressive properties, 90° tensile strength, interlaminar shear strength (ILSS), and 0° Izod resistance were measured and were related to the IFSS. On the 0° compressive test, delamination was not the dominant failure mode and hence the compressive strength did not change with IFSS. ILSS was measured by double notch shear method, and was proportional to the IFSS. 90° tensile strength increased with increasing IFSS, and it saturated because of the transverse fiber failure. The results of other properties were also discussed.