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Quadrature demodulation is used in medical ultrasound imaging to derive the envelope and instan-taneous phase of the received radio-frequency (RF) signal. In quadrature demodulation, RF signal is multiplied with the sine and cosine wave reference signal and then low-pass fi ltered to produce the base-band complex signal, which has high computational complexity. In this paper, we propose an e?cient quadrature demodulation method for B-mode and color fl ow imaging, in which the RF signal is demodulated by a pair of fi nite impulse response fi lters without mixing with the reference signal, to reduce the computational complexity. The proposed method was evaluated with simulation and in vivo experiments. From the simulation results, the proposed quadrature demod-ulation method produced similar normalized residual sum of squares (NRSS) and velocity profi le compared with the conventional quadrature demodulation method. In the in vivo color fl ow imaging experiments, the time of the demodulation process was 5.66 ms and 3.36 ms, for the conventional method and the proposed method, respectively. These results indicated that the proposed method can maintain the performance of quadrature demodulation while reducing computational complexity.