The effects of particle-size distribution on the longitudinal dispersion coefficient ( in packed beds of spherical particles are studied by simulating a tracer column experiment. The packed-bed models consist of uniform and different-sized spherical particles with a ratio of maximum to minimum particle diameter in the range of 1-4. The modified version of Euclidian Voronoi diagrams is used to discretize the system of particles into cells that each contains one sphere. The local flow distribution is derived with the use of Laurent series. The flow pattern at low particle Reynolds number is then obtained by minimization of dissipation rate of energy for the dual stream function. The value of is obtained by comparing the effluent curve from large discrete systems of spherical particles to the solution of the one-dimensional advection-dispersion equation. Main results are that at Peclet numbers above 1, increasing the width of the particle-size distribution increases the values of in the packed bed. At Peclet numbers below 1, increasing the width of the particle-size distribution slightly lowers D-L.