Abstract: Critical simulations of model hypoeutectic alloys were carried out with the KKSO multi-phase field model to study the lamellarrod eutectic transition. The result shows that the lamellar-rod eutectic transition does not appear abrupt but occurs over a range of compositions. Under the condition of isotropic interface energy, when the volume fraction of the minor phase is far lower than 1/π, the lamellar-rod transition occurs, and a rod microstructure forms. When the volume fraction of the minor phase is near to 1/π, the lamellar-rod transition does not occur completely, and it occurs later with the volume fraction of the minor phase increasing. When the volume fraction of the minor phase is far larger than I/π, it does not occur any longer. The simulations also show that the transition is related with initial lamellar spacing, which is in accordance with the experiment results qualitatively.