This study presents the effects of the orifice to pipe diameter ratio (defined as the beta ratio) on the flow field behind a thin circular square-edged orifice plate. We adopted a planar particle image velocimetry (PIV) system using two side-by-side cameras to measure velocity fields of a large area covering the reattachment region. The core, recirculation, and axisymmetric shear-layer regions are first suitably defined, and the characteristic length and velocity scales in different regions are then determined. When the orifice step height and mean streamwise velocity at the vena contracta are selected as the characteristic length and velocity scale, respectively, the mean flow field in the recirculation region shows nearly no beta-ratio dependence. When the orifice pipe radius and local maximum mean streamwise velocity in the core region are selected as the characteristic length and velocity scales, respectively, the local peak Reynolds stresses are found to be independent of the beta ratio in the shear-layer region close to the orifice plate. However, a beta-ratio dependence appears as the flow progresses, and the pipe wall increasingly affects the shear-layer region. Although the mean streamwise velocity profiles in the shear layer show self-similarity, the Reynolds stresses do not maintain a self-similar property, indicating that the normalized statistical quantities at different streamwise locations follow the same profiles in the transverse direction. (C) 2016 Elsevier Ltd. All rights reserved.