A numerical model has been established to calculate the primary spacing of cellular or dendritic structure with fluid flow considered. The computing results show that the primary spacing depends on the growing velocity, the temperature gradient on the interface and fluid flow. There is a critical growing velocity for the cell-dendrite transition, which has a relationship with the temperature gradient: Rcr = (3-4)×10-9GT. Fluid flow leads to an increase of the primary spacing for dendritic growth but a decrease for cellular growth, resulting in an instability on the interface. A numerical model has been established to calculate the primary spacing of cellular or deritritic structures with fluid flow considered. The computing results show that the primary spacing depends on the growing flow, the temperature gradient on the interface and fluid flow. velocity for the cell-dendrite transition, which has a relationship with the temperature gradient: Rcr = (3-4) × 10-9GT. Fluid flow leads to an increase of the primary spacing for dendritic growth but a decrease for cellular growth, resulting in an instability on the interface.