In this paper an experimenatal investigation of characteristics of developing ransitional steady flows in a square-sectional $180^{circ}$ curved duct is presented, The experimental study is carried out to measure axial velocity profiles secondary flow velocity profiles and entrance length by using Laser Dopper Velocimeter(LDV) system. The flow development is found to depend upon Dean number and curvature ratio. Of special interest is the secondary flow generated by centrifugal effects in the plane of the cross-section of the duct. The secondary flows becomes strong from $120^{circ}$ of bended angle on the duct. The entrance length of transitional steady flow is obtained to $120^{circ}$ of bended angle of the duct in this experimental conditions.