Phase-field simulation of the dendrite growth of an Fe–0.15 mass%C binary alloy with fluid flow was carried out, and the mechanism of deflection of dendrites in the alloy system was examined. In the simulation, the primary arms growing in a flowing melt inclined toward the upstream direction, and the deflection angle increased with increase in flow velocity. Decrease in deflection angle with increase in growth velocity of the dendrite tip and accelerated growth of side branches were also observed in the simulation. These results of simulation were in good agreement with experimental results. The simulation showed that the change in the thermal field has little effect on the deflection and that the change in the solutal field is the main factor responsible for the deflection of a dendrite in an alloy system. The maximum deflection angle of a single dendrite in the simulation was less than 15^°. The large deflection angles of grains (more than 20^°–30^°) in the experiments were thought to have been caused by nucleation in front of the dendrites and the subsequent competitive growth.