Abstract: We conducted an investigation into the plant assemblage of the Yellow River’s mainstream between September 16th and October 31st,2008.The total of 34 C₄ plant species were recorded,belonging to 9 families and 24 genera,which corresponded to 27%,28% and 25% of the total species,families,and genera respectively.Further analyses indicated that:(1)Gramineae and Cyperaceae were the dominant taxa in the area,contributing 61% of the C₄ plants in species number.(2)The number of C₄ plant species increased from the upper to the lower reaches of the river,while the standing crops and the C₄/C₃ ratio in the middle reaches were higher than those in the upper and lower reaches.(3)The number of C₄ plant species and C₄/C₃ ratio were correlated with altitude,annual air temperature,precipitation,and discharge,while the biomass of C₄ plants only had a significant correlation with annual precipitation.(4)The C₄/C₃ ratio in the reservoir region was higher than that in adjacent regions,which suggests this ratio may be an indication of vegetation dynamics under the disturbance of reservoirs.