Radio frequency (RF) transmission systems with high-precision phase stability are required by the next generation of particle colliders and light sources. An RF transmission system was designed to meet this require-ment. In this system, RF signal generated at the sending end is modulated onto a continuous wave (CW) optical carrier, transmitted through an optical fiber, and demodu-lated at the receiving end. The phase drift is detected by a digital phase monitor with femtosecond-level accuracy and compensated by a motorized optical fiber delay line (ODL). The measurement results show that the long-term phase drifts can be stabilized to within 100 fs (pk–pk), 500 fs (pk–pk), and 1.8 ps (pk–pk) in a 400-meter-long optical fiber over 1 h, 24 h, and 10 days, respectively.