A simple cavity ring-down technique employing a cw broadband diode laser is described for the reflectivity measurements of highly reflective mirrors. Due to the broad line width of the diode laser, the laser beam can be continuously injected into the ring-down cavity without tuning the cavity length or the laser frequency. Both Fourier- and time-domain data-processing approaches are developed to determine the reflectivity. In the Fourier domain, the amplitudes and phase shifts of the first and third harmonics of a periodic ring-down signal are measured as a function of the modulation frequency covering an appropriate range. The cavity decay time and the reflectivity of the cavity mirror are determined by minimizing a mean square variance containing both the amplitude and phase-shift error terms. On the other hand, in the time domain, the cavity decay time and the instrumental response time are determined simultaneously by fitting the measured waveforms of the cavity ring-down signals to a rigorous time-domain model via a multi-parameter fitting procedure. The reflectivity measurements are repeated at four cavity lengths and all the results obtained with both data-processing approaches are in excellent agreement. The reflectivity of cavity mirrors near 830 nm is statistically determined to be 0.99797 with an uncertainty less than 3 x 10(-5).