The longitudinal dispersion coefficient is usually computed directlyby empirical formulas or indirectly through introducing empirical formulas during the solution tovelocity distributions, which cannot fully reflect the physical mechanisms of dispersion. Based on the one-dimensional governing equation of river dynamics, a method to calculate longitudinal dispersion coefficient involving a power series solution to velocity distributionwas proposed. This formulation has been extended to the applications in 104 natural rivers with simplified rectanglecross-sections. The computed values are in the same order of magnitudes with the measured ones, indicating acceptable results.