This paper presents a phasor measurement algorithm for three-phase systems based on a modified phase-locked loop (PLL) system and presents the results of numerical simulations that show its performance with respect to different signal scenarios. The modified PLL system is first designed to estimate the phasors of the individual phases, frequency, and rate of change of frequency of the input signal. The positive-sequence phasor is then calculated based on the information provided by the PLL. Various simulations show that the proposed method exceeds most of the requirements stated in the IEEE Standard for Synchrophasor Measurements for Power Systems for a phasor measurement unit (PMU) of Class P.