The Young’s modulus of micromechanical silicon films is very different from bulk silicon structures. It is very meaningful to test Young’s modulus of micromechanical silicon films in related studies. Owing to its small and fragile characterstics, a suitable non-contact and nondestructive method is needed to test the Young’s modulus of micromechanical silicon films with plate-like and beamlike structures. Utilizing acoustic excitation and optical detection method based on theoretical analysis, a new resonant method is presented to determinate Young’s modulus of micromachined silicon films. By using this novel method, more accurate results have been achieved and the measuring error is 11%, better than the reported 14%.