In order to evaluate the effects of flow field on corrosion rate due to flow accelerated corrosion (FAC), the corrosion rate and flow velocity near wall were measured after an orifice In this study, the difference of flow field downstream from orifices with some aperture ratios was examined using Laser Doppler Velocimetry (LDV) Velocity distributions along the circumferential direction and along the flow direction with each aperture ratios of 0.62, 0.5 and 0.41 were compared, in order to verify the numerically simulated result The diameter of pipe was 50 mm, and the each orifice diameter was 31.0 mm, 25 mm and 20.5 mm, respectively Dependence of orifice aperture ratio on flow field downstream from an orifice was investigated by measurements of streamwise component of time-averaged velocity and standard deviation of velocity downstream from orifices