Loadbearing structures are the most common structural components in highend equipment. The loading and stress concentration easily initiates cracks, leading to structural failure. Meanwhile, the realtime monitoring of strain is essential for the health evaluation of structures. In this paper, the influence on the natural resonant frequency by the size of the Rectangular Microstrip Patch Antennas (RMPA) is revealed, based on microwave nondestructive testing technology. Furthermore, the interaction mechanism between the resonant frequency of RMPA and the strains is explored. RMPA can radiate at one or two resonant frequencies. The resonant frequency shifts as the length or width of RMPA varies. Thus, when a tensile stress is applied on the grounded metal sheet of RMPA, the geometrical dimension changes, which induces the resonant frequency shift. This phenomenon can predict the strain of the grounded metal sheet inversely. This is of great importance to the strain and crack monitoring of key loadbearing structures.