we experimentally demonstrate ultra-low pump power wavelength conversion based on four-wave mixing in a silicon racetrack-shaped microring resonator. when the pump and signal are located at the resonance wavelengths, wavelength conversion with a pump power of only 1 mw can be realized in this microring resonator because of the resonant enhancement of the device. however, saturation of the conversion efficiency occurs because of the shift of the resonance peak, which is caused by the change of the effective refractive index induced by a combination of thermal and free carrier dispersion effects, and it is demonstrated that the thermal effect is the leading-order factor for the change of the refractive index. the maximum conversion efficiency of -21 db is obtained when the pump power is less than 12 mw. this ultra-low-power on-chip wavelength convertor based on a silicon microring resonator can find important potential applications in highly integrated optical circuits for all-optical signal processing.