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There are two hypotheses in the leader progression model to illustrate the insulator breakdown process. One is the streamer onset time is relatively small that can be ignored. The other is the leader velocity at both sides of the composite insulator strings are equal. These assumptions are mostly based on the experimental results in rod-rod air gaps. In this paper, experiments were performed to study the breakdown process of a 220-kV composite insulator string with grading rings under positive short tail lightning impulses. The discharge current at the high-voltage side and the high-speed photographs were recorded synchronously. The method of identifying four typical stages of the breakdown process from the current waveform was proposed. The experimental results show that the streamer onset time should not be ignored when calculating the breakdown time, especially for the composite insulator string with grading rings. By fitting the experimental results, the relationship between the streamer onset time andUP/U ${}_{\text{50}}$ has been given. The results also indicate that the velocity of the positive leader is nearly 1.3 times than that of the negative leader during the breakdown process when the positive impulse is applied to the composite insulator string. [ABSTRACT FROM AUTHOR]