We have reported a novel optical sensor based on whispering gallery mode (WGM) resonance for measuring thermal deformation in microelectromechanical systems (MEMS) devices. New asymptotic expressions for transverse electric and transverse magnetic waves are developed based on electromagnetic theory derivations for the large size parameter ( times diameter divided by wavelength of light) limits. The optical thermal deformation sensor is characterized both theoretically and experimentally by considering the fact that the size parameter of the microspheres is very large at optical wavelengths. As a prototype thermal deformation sensor, an optical fiber experimental setup with tunable laser diode has been used for realizing the effect of WGM resonances due to change in surrounding temperature of a dielectric microsphere made of BK-7 glass. The quality factor of experimental resonance spectra observed in the laboratory is calculated approximately on the order of , which is sensitive enough for detecting micro or nano level deformation changes in the surrounding medium. The novel optical sensor can measure the thermal deformation in the MEMS devices as small as the submicron or nanometer level. This sensor could potentially be used for nanotechnology, MEMS devices, biomedical applications, and other microdevices.