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We have investigated impulse partial discharge (PD) and breakdown (BD) characteristics of a needle-plane gap in N2/SF6 gas mixtures under positive lightning impulse voltage application, and discussed their physical mechanisms. The 50% probability PD inception voltage (PDIV50), leader discharge onset voltage (LOV) and BD voltage (BDV50) were measured and analyzed as a function of gas pressure and SF6 content. Experimental results revealed the stepwise propagation process of the impulse PD and enabled us to classify the impulse PD in N2/SF6 gas mixtures into two types, the streamer discharge and the leader discharge. We also discussed the impulse PD propagation mechanisms in terms of PD parameters such as propagation length, time interval and current pulse magnitude, and suggested a sequential relationship in the PD propagation process under non-uniform electric field.