The multi-phase field model of Nestler is used to simulate the free growth of isothermal binary eutectic and to study the growth mechanisms under different initial nucleation conditions, including the given quantity and morphology of nucleus and the stochastic nucleation. The variety of Avrami exponent calculated through the relationship between solid volume fraction and transition time demonstrates that there are three possible eutectic growth mechanisms, i.e. the diffusion limited growth, the lamellar growth and the spinodal decomposition growth. And the process of free eutectic growth is the transition process of the three growth mechanisms from one to another.