In this paper, we investigate the effect of the stochastic randomization in the on-ramp system using the cellular automata traffic flow model. Both single-lane main road and two-lane main road situations are studied. The variation of the phase diagram with the randomization probability p is studied. A new phase, i.e., the maximum flow phase is reported in the two-lane main road situation. The capacity of the on-ramp system under different p and vmax is discussed.