This study investigates the dynamic response of an electrostatically defected capacitive cantilever Functionally Graded (FG) micro-beam subjected to a harmonically varying thermal load, which incites vibration due to the different material properties of the beam constituents and the thermo-elastic coupling effect. The FG beam is made of a mixture of metal and ceramic, where the material properties vary continuously through the thickness according to an exponential distribution law (E-FGM). Assuming the Euler- Bernoulli beam theory and the infinite speed of heat transportation, the equation of motion and the conventional coupled energy equation are derived. Applying Galerkin formulation and then using the Rung-kutta method as an efficient numerical tool, these equations are simultaneously solved. Changing the ceramic constituent percentage of the bottom surface, five different types of FGM micro-beams are investigated and the results are presented for all types. Numerical results show the response of a cantilever FG microbeam subjected to a harmonically varying temperature input. Moreover, the influences of the beam ceramic constituent percentage on stability, vibrational behavior and natural frequency are presented.