We demonstrate an ultra-narrow-band optical filter based on a hybrid-microsphere consisting of a coated SiO2 microsphere. As compared to the SiO2 microsphere, the hybrid-microsphere produces a quality factor of >108, a transmission spectrum bandwidth of <1 pm, and an increased side-mode suppression ratio. The microsphere surface roughness and whispering gallery mode (WGM) transmission spectra are measured experimentally. A 0.01 nm bandwidth, single-wavelength fiber laser output is achieved with a tunable wavelength, using the SiO2 microsphere as the mode selector. The optical field distribution and WGM transmission spectrum of the hybrid-microsphere with different coating parameters are theoretically investigated by the finite-difference time-domain method.