The importance of the rivers in meeting human needs and also growing pollution of surface waters, studying of mixing processes for recycling contaminated water could be important for management of resources. Due to importance of bed form such as dunes and also vegetation in rivers, channels and streams, the Study of pollutant mixing coefficient on the interaction of Dune and vegetation is more important than before. The purpose of this study is to investigate transverse mixing coefficient and structure of flow on dunes in the presence of vegetation in walls. To check mixing coefficient, experimental measurements were made over fifth to ninth dunes in a series of ten of 2-dimensional asymmetric gravel dunes in two cases with and without vegetated walls. The dunes had an angle of lee slope of 28?, a mean wavelength of 1 m and two mean height of 0.04 and 0.08 m. The banks of the flume were covered by rice stems with median diameter of 2.7 mm and stem density was calculated 400 stem/m. The used sand for construction of dunes had the mean diameter of 14mm. to investigate the effect of aspect ratio on transverse mixing coefficient, five flow depths were used in two cases with and without vegetation on walls. Concentration data in five sections by EC meter at the downstream of the injection site were taken. 27 samples of concentration were taken at each sampling section. To investigate of structure flow on dunes, samples of velocity at 14 sections were taken. The results show that dunes are importance in increasing transverse mixing coefficient to the extent that this coefficient, in presence of dunes is 2.36 more than that on flat bed and increasing height of dunes makes this value be 3.19 more times than that on flat bed. Despite the increase in the transverse mixing coefficient due to interaction of Dune and vegetation, the effect of vegetation on this coefficient is less than that of Dune. Due to the constant channel width in both cases with and without vegetation, with increasing aspect ratio, secondary currents become weaker and transverse mixing coefficient decreases. Length of complete mixing is also reduced with increasing depth. However, when the be is flat, the distance the tracer needs to go along the channel should be 35 meter in order to complete mixing can be achieved. This length on dunes with two heights of 4 and 8 cm is equal to 10 and 11 meters, respectively that is 1/3 that on flat bed. Increasing height of dune transfer flow separation zone from nearness of crest of dune with height of 4cm to downstream of lee slope for dune with height of 8cm. Increasing height of crest of dune, due to the maximum stress, occurs farther from the bed. The maximum turbulence intensity on dunes with height of 8cm occurred at top of separation zone in wake region and was 0.1 m/s. With decreasing height of dune, maximum intensity of turbulence decreased and reached 0.0898 m/s on stoss lee.