This study aims to better understand the characteristics of the tidal- current profiles and the near-bed boundary layer structures off the southwestern coast of Taiwan. The velocity profile is measured by a bottom-mounted ADCP. Six experiments were conducted, each lasted 10~20 days and the water depth ranging 12~18 m. Twenty-minute averaged velocity profiles have been fitted to a logarithmic form with 4% accuracy. The friction velocity (u*) and roughness length (z0) are then derived from the slope and intercept of the best-fitted straight lines. Our results show that the profile shape and friction velocity vary tidally, the latter reaches O(0.06)ms-1 during peak current flow. The magnitude of z0 is large and scattered, but it shows a general trend of decrease with increasing flow speed. The observed log-layer height increases, and the bottom drag coefficient (CD) decrease, respectively with increasing flow speed. Measurements also show that water turbidity increases with rainfall, as a result the z0 and CD also increase. Finally, harmonic analysis of the tidal currents indicate significant changes between winter (homogeneous) and summer (stratified) conditions. In winter the vertical variation of orientation and phase is small, whereas in summer there was a 150 orientation and 250 phase difference (the bottom currents lead the surface currents) between the near surface and near bed regions.