Abstract: As compared to the phasor measurement units (PMU) installed in power grids transmission line, the PMUs in distribution level have to perform single measurement, and suffer from stronger harmonics and noise pollution. In this paper, the low-pass digital differentiator was optimally designed for PMU frequency measurement in distribution level. The optimal digital filter aimed to possess ideal differentiator frequency response within phasor frequency band, and to gain perfect harmonic suppression and minimized noise in high frequency band. By solving well-defined constrained optimization problems, the linear phase finite impulse response (FIR) digital differentiators were designed with the filter lengths from 1 nominal fundamental cycle to 1.5 cycles. The simulation results shows that the designed filters, working after the synchrophasor estimator recommended in IEEE C37.118.1, perform fast and accurate frequency measurement both in steady and dynamic conditions in distribution level.