Shafts made of carbon fiber composite have higher fundamental bending natural frequency when the stacking angle of fiber is close to zero from the axial direction compared to that of metals such as aluminum and steel, which is an important property for power transmission shafts. However, the small stacking angle not only reduces the torque transmission capability of composite shafts but also makes the joining of composite shafts to other materials difficult. If a shaft is made of both carbon fiber composite and metal such as aluminum, the bending natural frequency as well as the torque transmission capability of the shaft can be increased: the carbon fiber composite increases the bending natural frequency and the aluminum increases the torque transmission capability. In this paper, a hybrid shaft was manufactured by co-curing carbon fiber epoxy composite to an aluminum shaft to increase the bending natural frequency and damping without reducing the torque transmission capability of the shaft. In order to reduce the residual thermal stress at the interface of the two materials, the aluminum shaft was compressed by giving a compressive preload during co-curing operation. From the fatigue test and finite element analysis of the co-cured hybrid shafts, it was found that the compression by a preload increased the torque transmission capability of the hybrid shafts.