Abstract: In this paper theoretical analysis has been made on the relation between the primary spacing of solidification microstructures and the solute Peolet number. It is verified that the maximum of the primary spacing corresponds with the minimum of Peolet number. Prom this a oriterion for cell-to-dendrite transition has been put out which can be writion as V (cr)=2DG L/mC 0（1-k） Experimental results of the cell-to-dendrite transition in Al-4.5(wt)%Cu are favorable to the oriterion.

Key words: Peclet数, 胞枝转变