The effect of different intensities of vocal emotion on event related potentials has yet not been studied. We therefore investigated 16 healthy participants with emotion and sound decision on neutral and happy voice which varied continuously in intensity. The result found that neutral and happy voice can be differentiated on P2 component under both explicit and implicit condition. Moreover, the P2 parameters were linear correlated with the rate of happiness, suggesting a graded processing of vocal emotion in early stage. However, the brain distinguished neutral from happy in P3 interval when performing explicit task but exhibit a categorical feature.