This paper proposes theoretically a novel terahertz source based on the conventional single-mode optical fibers. Utilizing four-wave-mixing effect in optical fibers and the dual-wavelength laser as the pump source,a compact,wide-tunable countinuous wave (CW) terahertz source is constructed. The four-wave mixing process has been discussed in detail through the coupled-wave theory,and the analytical expression of terahertz output power is obtained. It is shown that when the pump power is 1 kW,the terahertz peak power is about 60 mW. This scheme provides a possibility to realize a high-power and compact CW terahertz source.