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.　©　2017,　Science　Press.　All　right　reserved.