The purpose of this study was to design a passive nanostrain Radio Frequency strain sensor that helps to monitor the strain changes caused by traffic in motion on bridges. The phase sensitive interrogation method was applied meaning that the strain changes will be measured by the cavity sensor phase shift. The results revealed that the RF strain sensor could achieve a resolution of a few nanostrain. The principle conclusion was that the designed RF strain sensor has nanostrain sensitivity. Coaxial-Cylinder sensor sensitivity was 8 nanostrain. Cylinder volume resonant cavity sensor sensitivity was 8 nanostrain for high Q and 4 nanostrain for low Q. (BW = 160Hz) These sensitivities were somewhat larger than theoretical estimates due to noise from sauces other than the thermal noise used in the theoretical estimation. Therefore sensors will be useful for Structural Health Monitoring applications.