A novel frequency measuring algorithm for the power system is presented based on a model with linear current amplitude during a brief time. The frequency measuring algorithm provided is used to obtain accurate instantaneous parameters for the power system. In the algorithm， the amplitudes of four consecutive sampling currents are supposed to be linear. After rigorous mathematical deduction， including Taylor series expansion and the geometric proportion theorem， the instantaneous frequency can be derived. The errors due to the linear amplitude model are analyzed qualitatively and those of the A/D converter and Taylor series expansion are considered. By the proposed technique， without any digital filters， the measured frequency can be derived easily， quickly and accurately. The simulation results show that swing or not， gentle or violent， the algorithm proposed is highly accurate and easily adapted to the random noise in the current.