A simple, accurate and reliable method for measuring the reflectivity of laser- grade mirrors (R > 99:5%) based on cavity ring-down (CRD) technique has been success- fully demonstrated in our laboratory using a pulsed Nd:YAG laser. A fast photomultiplier tube with an oscilloscope was used to detect and analyse the CRD signal. The cavity decay times were measured for three cavities formed by a combination of three mirror pairs. The absolute reflectivities R₁, R₂, R₃ were determined to be 99.94%, 99.63%, 99.52% at normal incidence. The reflectivity of mirrors is measured to an accuracy of 0.01%.