This paper studies optimal designs for vibrating structures using a moving isosurface threshold method (MIST). In the present study, a combination of strain and kinetic energy densities is selected as a response function of natural frequency and then formulations to maximize a specific frequency, frequency separation, and average-mean are derived. An efficient algorithm is developed to find a moving isosurface threshold level for evolving the design boundary and updating the weighting factor. The present algorithm coupled with commercial finite element analysis (FEA) software is used to study optimal designs for vibrating structures. The obtained optimal designs are fabricated and the experimental tests are conducted to validate the optimal topologies.