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In this paper, a control strategy of the single-phase active filter using a novel d-q transformation is proposed. This d-q transformation is implemented by using Hilbert transform, by which the instantaneous single-phase voltage and current are converted into complex vectors (analytic signals) on the instantaneous basis. Since the fundamental components of voltages and currents are converted into DC components on the d-q coordinate both in the steady and transient states, the harmonics of voltages and currents can be obtained precisely through the low-pass filer installed on the d-q coordinate. This results in the economical active filter with a small capacitor suppressing the DC bus voltage fluctuation because the fundamental power can be fully removed from the active filter input.