Abstract: An optical feedback cavity ring down (OF-CRD) technique for simultaneously measuring the S-(perpendicular to the plane of incidence) and P-(in the plane of incidence) polarization reflectivity of highly reflective coatings is developed. Polarized light with a certain ratio of S-and P-polarization power and with square-wave modulated intensity is coupled into a ring-down cavity. The ring-down signal that leaks out from a cavity mirror is recorded at the negative edge of the modulation and fitted to a bi-exponential function to determine simultaneously the ring-down time for S and P polarizations. The reflection coefficients of highly reflective coatings for the S and P polarizations are therefore calculated from the determined ring-down time. Compared to the results obtained with purely S or P polarization, the differences for both S and P polarizations are respectively 1 ppm and 15 ppm, indicating the correctness of the simultaneous measurements. Compared to the conventional CRD approach, this simultaneous measurement method has the advantages of simpler configuration, easier operation, and higher speed.