Excellent sensitivity and accuracy in the measurement of deformation occurring in materials tests have been achieved with a newly developed microwave frequency sensor and instrumentation system. The strain sensor comprises a microwave cavity resonating in the circular TE113 and TM110 modes. Detection of axial strain occurs due to the changes of resonant frequency incurred by cavity length changes. Axial strain sensitivity for the TE113 mode was 6 × 10−6 per MHz, whereas radial dimensions of the cavity were related to frequency of the TM110 mode. Aperture coupling of the cavity to the end wall of Ka band waveguide provided signal excitation of the two monitored modes. Phase-locked frequency stability enabled digital count/display of resonant frequencies to within 70 kHz at 35 GHz. Room temperature tension test results demonstrated a strain measuring sensitivity (±1 × 10−6) and accuracy (±1% of the measured value) equivalent to those of electrical resistance strain gauges. The system yields accurate measurements of elastic strains as well as small departures from elastic response and hysteresis behavior during unloading and reloading. Creep test results confirm that measurement sensitivity and accuracy are retained in elevated temperature tests. Strain response on loading, subsequent creep deformation, and creep recovery after unloading can all be measured in detail. The stability of microwave sensor calibration after exposure for 22 × 106 s at temperatures from 728 to 866 K is shown to be excellent.