A resonant infrared thermal sensor with high sensitivity, whose sensing element is a bimaterial structure with thermal expansion mismatch effect, is presented. The sensor detects infrared radiation by means of tracking the change in resonance frequency of the bimaterial structure with temperature attributed to the infrared radiation from targets. The bimaterial structure is able to amplify the change in resonance frequency compared with a single material structure for a certain mode of vibration. In accordance with the vibration theory and the design principle of an infrared thermal detector, the resonant sensor, which can be arranged in an array, is designed. The simulation results, by using finite element analysis, demonstrate that the dependence of resonance frequency on temperature of the designed structure achieves 1 Hz/10 mK. An array of 6×6 resonant thermal sensors is fabricated by using microelectronics processes that are compatible with integrated circuit fabrication technology. The frequency variation corresponding to the temperature shift is obtained by electrical measurement.