Carbon fiber (CF)-reinforced polyimide (PI) has been widely used in many engineering fields because of its high specific strength and stiffness. However, PI does not adhere well with CFs because it has low free surface energy. In addition, high viscosity in the melted phase causes poor impregnation. In this study, surface treatment methods, that is, coupling agents (CAs) with plasma treatment on CFs, were applied to increase the interfacial strength between the CFs and the PI matrix. The modified CF surfaces were analyzed by X-ray photoelectron spectroscopy and scanning electron microscopy. To analyze the effectiveness of the surface treatment method, the interlaminar shear strength (ILSS) was measured using the three-point bending test. From the test results, the ILSS of the specimens treated with the silane CA after the plasma treatment increased by 48.7% compared with the untreated specimens.