In this paper, a new camshaft phase adjustment mechanism with twin harmonic drives, which can be adopted to DOHC engines, is described and illustrated. The mechanism consists of an input circular spline rigidly connected to the camshaft sprocket, a flexspline, an output circular spline attached to the camshaft, a fixed wave generator, a rotatable wave generator, and a stepper motor. The angular position of the output circular spline relative to the input circular spline, which determines the camshaft phase, may be changed or kept constant by controlling the stepper motor to rotate the rotatable wave generator. The advantages of the mechanism in practical use include: multistage phase adjustment with a small interval over a wide range, small size of the rotating part along the axis direction, small moment of inertia, and convenience of wiring.A prototype of the design for a DOHC engine was built. Phase adjustment tests show that the mechanism can lock reliably or adjust camshaft phase accurately according to the requirement. Also, reliability of the mechanism has been examined in a 200 hours preliminary test. Improvement in volumetric efficiency of the engine with the mechanism was measured and simulated. In the range of test engine speed, volumetric efficiency increased by 4.5 to 10.5 percent.