An ultra-small disk resonator consisting of a suspended silicon disk with a submicron bending radius sitting on an SiO(2) pedestal is demonstrated experimentally. An asymmetrical suspended rib waveguide is integrated as the access waveguide for the suspended submicron disk resonator, which is used to realize an ultra-small optical sensor with an improved sensitivity due to the enhanced evanescent field interaction with the analyte. The present optical sensor also has a large measurement range because of the ultra-large free-spectral range of the submicron-disk resonator. As an example, a suspended submicron disk sensor with a bending radius of 0.8 μm is designed, fabricated, and characterized. The concentration of NaCl aqueous solution and organic liquids is measured with the suspended submicron-disk sensor, and the measured sensitivity is about 130 nm/RIU, which agrees well with the simulation value.