The occurrence and development of scour hole around bridge pier are main causes of bridge destruction. The first step to solve this problem, is investigation of flow pattern and recognition of scour process around bridge pier. The prediction of occurrence, development and final situation of scour hole around pier is the most important in the hydraulic design of bridges. Although the flow pattern around a single pier has been widely investigated, there are few studies around pier groups. Moreover the scour hole around group pier is affected by factors which are not valid for single pier.The present study contains results of the experiments on the flow pattern around two and three-circular piers situated in a line arrangement, measured by an Acoustic Doppler Velocimeter (ADV) under a clear-water scour condition. These group piers were tested with s/b=4, 6 and 8, and compared with single pier. Laboratory flume experimental detection were made by using ADV to determine velocity, Reynolds stress distribution and turbulent intensities. Experiments were carried out in a horizontal re-circulating flume in the Hydraulic Laboratory at the Isfahan University of Technology in Iran. The flume had 8 m long, 0.4 m wide and 0.6 m deep. Uniformity graded sand of a median diameter, d, of 0.71 mm and the geometric standard deviation 1.2 was applied. Scour hole waz frozen by cement to facilitate performance of the experiments. Results of this experiments, indicated that in group of two piers during the experiment, scour depth of first pier is greater than that of the single pier due to the reinforcing effect, and because of sheltering effect scour depth of the second pier is less than that of the the single pier. In group of three piers the scour depth of the second pier is equal to the single pier due to the interaction of reinforcing and sheltering effect, and the equilibrium scour depth for the first and the third piers is equal to front and rear pier in group of two piers respectively. By increasing of space between piers, reinforcing and sheltering effects reducing. In upstream, the velocity profiles and turbulent intensities showed that the existence of primary vortex associated with down flow inside the scour hole and in downstream, wake vortex caused irregularities in flow pattern. In the upstream and scour hole continuity to 7.6 cm distance from the up stream, the values of velocity component in x directio were negative which indicated the flow separation. Whereas, in the down stream negative value of u were observed just in rear pier and in surface water. The RMS u and u-velocity profiles showed that osilliatons of u velocity in rear pier causes the u velocity reduces in this region, and wake wortex is the main factor for osilliation of u velocity (root mean square u). The values of root mean square u in front of second and third piers are distance between different space of piers and clearly indicated that root mean square u in 4b and 6b spaces are greater than that 6b and 8b spaces, respectively. Key words : scouring, bridge pier, group piers, flow pattern.