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Abstract: In order to further understand the initiating mechanism of positive cloud-to-ground flashes, the characteristics of initial breakdown pulse trains are investigated and compared by using the data of electric field observed in Beijing and Guangzhou. According to the difference in initial polarity between initial breakdown pulse and the first return stoke, three types of initial breakdown pulse trains, including the same polarity (type I), the opposite polarity (type II) and the composite polarity (type III), have been identified. The categories of initial breakdown pulse trains in Beijing are the same as those in Guangzhou. However, in Beijing and Guangzhou, the percentages of the types are obviously different, which may be caused by different charge configuration. In Beijing, the percentages of type I, type II and type III are 55%, 39% and 6%, respectively. In Guangzhou, the percentages of type I, type II and type III are 81%, 15% and 4%, respectively. By comparsion of the pulse train parameters, it can be found that the width of individual pulse and the interval of adjacent pulses are larger in Guangzhou than those in Beijing. The difference may be caused by different discharge distance. At the same time, both in Beijing and Guangzhou, the value of pulse train duration and the ratio of the maximum peak amplitude of initial breakdown pulse train to the amplitude of first return stroke peak for type II are smaller than those for type I. However, the value of the interval between initial breakdown pulse train and first return sroke for type II is larger than that for type I. We also find that, for type II, the ratio of the maximum peak amplitude of initial breakdown pulse train to the amplitude of first return stroke peak is obviously larger in Beijing than that in Guangzhou. We believe that the intensity of initiation breakdown process as type II is larger in the region with higher latitude. It can be speculated that the intensity of initial breakdown pulse train preceding positive first return stroke is further affected by the strength of lower positive charge region. Some possible causes of the differences have been discussed. [Copyright &y& Elsevier]