Abstract: A new optimal sensor placement method, called sensitivity analysis method (SAM), is presented in this paper. It analyzes the sensitivity according to the changes of structural mode shape due to stiffness changes (damages are reflected through stiffness reduction). Two traditional methods of optimal sensor placement, the effective independence method (EFI) based on maximizing the data information and the kinetic energy method (KEM) based on maximizing the kinetic energy, are discussed briefly. And we compare the new method (SAM) with the two traditional methods under two criteria: mean square error (MSE) and modal assurance criterion (MAC). The numerical simulation results show that the SAM is the best of the three.