CdS nanowire (NW) ring cavities were fabricated and studied for the first time. The rings with radii from 2.1 to 5.9 microm were fabricated by a nanoprobe system installed in a scanning electron microscope. Radius dependent whispering gallery modes (WGMs) were observed. A straight CdS NW with Fabry-Pérot (F-P) cavity structure was fabricated and placed by the side of a NW ring cavity to form a coupled ring-F-P cavity. When the NW ring was excited by a focused laser, a bright green light spot was observed at the output end of the straight NW, indicating that the latter had served as an effect waveguide to couple the light out from the ring cavity. The corresponding light spectrum showed that the WGMs had been modulated. We confirmed that the NW F-P cavity had served as a modulator. Such a coupled cavity has potential application in a nanophotonic system.