As a structure is scaled down, the ratio of surface to volume increases and the surface effects thus stand out, which may result in the different material properties from those of bulk. Therefore, some key parameters such as Young’s modulus and adhesion energy, which concern the performance and reliability of the micro/nano-electro-mechanical systems (MEMS/NEMS), are unknown. Sometimes, the direct measurements of some parameters are extremely difficult if not impossible, for example, measuring the position of an adsorbate as small as a molecule or an atom. How to use the measured quantities such as resonant frequencies and deformation to determine those key parameters forms an inverse problem. Every inverse problem requires a particular approach. In this chapter, we present some effective approaches of solving the inverse problems in the MEMS/NEMS applications. The chapter consists of two major parts: the inverse problems in (1) micro/nano mechanical resonator and (2) the stiction test of MEMS/NEMS.