At a low velocity, the lamellarrod eutectic transition can be controlled by the volume fraction of one eutectic phase only. The factors which affect this kind of transition at high growth velocities are not clear. Based on the competitive growth rule, the criterion for lamellarrod transition is obtained by combining the models of lamellar and rod eutectic growth under rapid solidification conditions. It is shown that for a certain volume fraction, if its value fluctuates around the critical point predicted by the JH Jackson and Hunt model, the increase of the growth velocity or the partition coefficient will lead to the rodlamellar transition. Otherwise, no transition will take place. The lamellarrod eutectic transition at high growth velocity is controlled by the volume fraction variation, which is caused by the increase of the growth velocity.