The dendritic growth of an Al-3Cu-1Mg alloy was calculated using a three-dimensional cellular automaton model.Effects of the off-diagonal terms of the solute diffusion coefficient matrix on the dendritic growth during solidification of the alloy were investigated.The solute diffusion coefficients were treated in two ways:（1）by considering the concentration-dependent and temperature-dependent diffusion

and neglecting the effect of solute interactions or the off-diagonal terms of the diffusion matrix

or（2）by calculating the diffusion coefficients by solving the full diffusion matrix.The results indicate that the calculated dendrite tip selection parameter is the smaller than that obtained using the marginal stability criterion.When the off-diagonal terms of the solute diffusion coefficient matrix are considered

the growth velocity of the dendrite tip is decreased

while the dendrite tip radius is increased as a result of the attraction of the solutes in the Al-Cu-Mg system.