Using the molded LDPE specimen with the metal foil electrode, the initiation and growth of electrical trees generated by positive and negative impulse voltage were studied. The Weibull plots of the numbers of impulse voltage shots up to the tree initiation and to the puncture of the specimen were analyzed. The difference between the plots of positive impulse shots and those of negative ones demonstrates the difference between the initiation and growth of a positive impulse tree and those of a negative one. Taking account of the results of the electric field calculation in the vicinity of the electrode, the tree initiation mechanism was examined. The difference of the tree initiation between a positive impulse tree and a negative one is suggested to be due to the difference between the tree growth following the initiation process of positive impulse tree and that of negative one.