We report a high performance relative humidity (RH) microsensor based on a few-layer graphene oxide (GO) flake coated photonic crystal (PC) cavity. Since the GO layer is highly water-reactive and interacts with the evanescent cavity mode strongly, the exposure of the GO-PC cavity in varied humidity levels results in significant resonant wavelength shifts, showing a slope of 0.68 nm/%RH in the range of 60%-85%RH. By monitoring the power variation of the cavity reflection, the microsensor presents an ultrahigh sensitivity exceeding 3.9 dB/%RH. Relying on the unimpeded permeation of water molecules through the GO interlayers and microscale distribution of the cavity mode, the integrated sensor has a response time less than 100 ms, which promises successful measurements of human breathing. Combining with the ease of fabrication, this high performance RH sensor provides great potentials in applications requiring optical access, device compactness, and fast dynamic response.