Narrow micromachined bilayer cantilevers, which are broadly used to determine different thin film material properties, have rarely been used to characterize the Poisson's ratio. It is difficult to be determined from the tip deflection, and thus the Poisson's ratio, of the narrow bilayer cantilever. In this paper, the tip deflections of ultra-wide micromachined bilayer cantilevers carry the needed information for finding the Poisson's ratio of thin-film materials. The measurement process and its corresponding model, based on the plate theory, is introduced and tested. The Poisson's ratio of the thin film is determined by comparing the tip deflections of the bilayer cantilever before and after the deposition of the upper layer. Because the fabrication processes are widely used in surface micromachining, the method can be easily implemented.