Approaching flow to bed forms and hydraulic structures can be appeared as decelerating or accelerating in rivers and natural channels. Knowledge of incipient motion in non-uniform flows can greatly influence on scouring and resistance to flow in rivers. The objective of this research is to study the turbulent flow characteristics on the threshold of motion in non-uniform flows. The experiments were carried out in Hydraulic Laboratory of Isfahan University of Technology in 8(m) long, 0.4(m) wide and 0.6(m) deep flume using three sets of uniform sediment with median grain sizes of (d50=1.8, 3.8 0.95 mm) and three slope of 0.0075, 0.0125 and 0.015 under hy; decelerating flows. Turbulent flow characteristics and velocity components were measured in three different cross sections (5, 5.75 and 6.5 meters from the flume entrance) by Acousstic Doppler Velocimeter (ADV). Obtained data from 81 velocity profiles was analyzed and filtered by WinADV software and the Reynolds stress under different sediment particles and bed slopes were investigated. The results showed that the velocity distribution and the normal and shear stresses are affected by weakly mobile bed condition. In calculating the critical Shields parameter several factors such as the method of calculating the velocity of the object (The two methods were compared with basic logarithmic methods and boundary layer theory and that were acceptable consistency), the numerical constants of this methods and choice or not choice a data had considerable effect on Shields parameters. Results revealed that critical flow condition for uniform sediment motion in a decelerating flow depends on the relative submergence (the ratio of flow depth to grain size diameter of sediment particle. A revised Shields diagram relating critical stress, grain Reynolds Number was derived by considering the effect of relative submergence. Also, using dimensionless grain diameter instead of grain Reynolds number, incipient motion of sediment particles was evaluated for different bed slopes. Results of this study can be applied to the design of stable alluvial channels, and hydraulic models in river engineering. Keywords : Fluvial hydraulics; Open channel flow; River beds; Sediment traort; Shear stre Streamflow