A series of experiments on the wave front of nonlinear wave trains in deep water were conducted in a super long tank (300m∗5.2m∗5m) in Tainan Hydraulics Laboratory. The initially uniform and weakly modulated wave trains were generated with steepness ε=0.07∼0.32. The modulation of the wave front exposes a slow oscillation with an increasing trend during propagating downstream along the tank. Two different evolution types of the wave trains were observed, either the near recurrence of initial state or the onset of breaking in strong modulated wave train, which depends on the initial wave steepness. The local maximum wave steepness increases sharply with the initial wave steepness and levels off for initial wave steepness larger than 0.16. The breaking criterion of wave group in deep water is much lower than that of Stokes wave. The group velocity of wave train propagation depends on the state of modulation of the wave front. Based on the experimental results, the estimated group velocity shows nearly 10% larger than the linear group velocity.