This paper describes the change of the residual AC breakdown voltages of 6.6kV dry-cured XLPE cables during an immersed accelerated aging test. The influence of bow-tie trees on the breakdown voltages was investigated in detail. The accelerated tests was carried out by applying 6.9kV (1kHz) at room temperature with the conductor strands and insulation shields kept constantly moist. In the breakdown tests, the initial voltage step was 35kV/hour and subsequent voltage step was 5kV/half hour. The residual breakdown voltage changes very much during the accelerated test. From the results of Weibull distribution plot on the AC breakdown voltages, the cause of the breakdown can be divided into three regions; Region A (35kV), Region B (35-90kV) and Region C (>90kV). To clarify the causes of each regions, we used the prebreakdown partial discharge detection method to determine the starting point of the breakdown. Electrical trees started from bow-tie trees (>600μm) touch the semiconductive shields in Region A and B. From these results, it is clear that the decrease of the breakdown voltage during the accelerated test is mainly caused by these bow-tie trees. In Region C, we conjecture that the breakdown voltage is reduced by bow-tie trees untouch the semiconductive shields. Furthermore, we investigated the influence of bow-tie trees touch the semiconductive shields on the breakdown voltage, it was found that the voltage changes by the moisture at bow-tie trees.