To meet the requirements of wide area harmonic phasor monitoring, harmonic phasor measurement algorithms should be elaborately designed. In this paper two-cycle long FIR bandpass filters are designed for fast and accurate harmonic phasor measurement, with the IEEE Std. C37.118.1 for a fundamental synchrophasor measurement being followed as the reference for formulation and evaluation of the proposed method. Quantitative relations are first derived between the filter frequency response and the measurement errors caused by the fundamental, the harmonic to be measured, the other harmonics and the wideband noise. Filter specifications are then defined and filter design criteria is formulated according to the relations and practical considerations. Finally the measurement filters are designed based on dynamic harmonic phasor model and weighted least square (WLS) technique. Simulation experiments have confirmed excellent performances of the proposed algorithms both under steady and dynamic conditions.