This paper describes a development of a new method (: Cantilever Method) to measure Young’s modulus of flexible materials. The method is based on a nonlinear deformation theory that takes into account large deformation behaviors. A set of testing devices was designed and machined. Measurements were carried out on two kinds of flexible materials (PVC: a high-polymer material and SWPA: a steel material). The modulus measured by this method is “Secant modulus”. The results of my evaluation confirm that the new method is suitable for flexible thin plates or rods. Based on the assessments made the method can be further applied to thin sheet and fiber materials (e.g., steel belt, glass fiber, carbon fiber, optical fiber, etc.).