In the paper, the influence of aggregation structure and traps on both DC and impulse flashover voltage of semi-crystalline cross-linked polyethylene (XLPE) in vacuum are investigated. After heat treatment at 135 ℃ for 10 min, the XLPE samples were quenched to -56 ℃ or -25 ℃, cooled normally, cooled at a rate of 1 ℃/min or 05 ℃/min to room temperature, respectively. The electrical properties, microstructure and trap distribution of heat-treated samples were studied. Compared with untreated samples, it is found that the dc and impulse flashover voltages of heat treated samples are raised by 76% and 19%, respectively. It is proposed that the aggregation structure and trap level is changed by heat treatment, leading to the improvement of surface flashover properties in vacuum. It is suggested that the vacuum flashover characteristics can be improved by controlling the aggregation structure and trap levels of semi-crystalline polymers.