Blood flow measurement by using Doppler ultrasound has become a useful tool for diagnosing cardiovascular diseases and as a physiological signal monitor. Recently, a couple of pocket-sized ultrasound scanners have been introduced for portable diagnosis. However, it is not convenient to apply Doppler measurements by commercial duplex scanner during exercise or sleeping due to the shape of handheld probe. In addition, most devices use the digital integrated circuits to process the Doppler signals from flowing blood. However, some attached components are needed such as RF and/or audio ADC, on board memory, programmable timer, and LCD graphic display module and its control circuit. In order to simplify these complex circuits, an implementation of a smartphone based portable ultrasound pulsed-wave (PW) Doppler flowmeter is reported in this study. A 10-MHz ultrasonic surface transducer was designed for the dynamic monitoring of blood flow velocity. The directional audio Doppler shift signals were obtained using a portable analog circuit system. After hardware processing, the Doppler signals were fed directly to an Android-based smartphone for Doppler spectrogram analysis and display in real-time. In order to verify the portable device, a Couette flow phantom was used to simulate the flowing blood for in vitro verification. Doppler spectrograms of porcine blood flow were measured using this stand-alone portable device under the pulsatile condition. Finally, the arterial blood flow of a volunteer carotid was measured in vivo to verify the performance of the smartphone-based portable PW Doppler ultrasound device. All of the results demonstrated the potential of using a smartphone as a novel embedded system for portable medical ultrasound applications.