Abstract: The paper extends a significant progress both in theory and in application of the prediction of longitudinal dispersion coefficient for natural rivers.The method is based on the hydraulic geometry relationship of stable rivers and on the assumption that the equations used in uniform-flow are still valid for local depth-averaged variables.The lateral distributions of the local flow depth and the deviation of local velocity from the cross-sectional average value are determined for straight alluvial rivers.By using the suggested transverse mixing coefficient equation and the direct integration of Fischer's triple integral,the paper presents a new theoretical equation of the longitudinal dispersion coefficient for natural rivers.The distinct feature of the new equation lies in its involvement of the transverse mixing coefficient.By the comparison with 59 sets of field data and the equations by other investigators,it is found that the equation presented in this paper predicts the longitudinal dispersion coefficient of natural rivers more accurately.Moreover,the new equation is reasonable in theory and clearer in dispersion mechanism as compared to other equations.