The dielectric breakdown of epoxy composites used for transformers was experimented and then its data were applied to Weibull distribution probability. First of all, speaking of dielectric breakdown properties, the more hardener increased, the stronger breakdown strength became at low temperature because of cross-linked density by the virtue of ester radical. The breakdown strength of specimens with filler was lower than it of non-filler specimens because it is believed that the adding filler forms interface and charge is accumulated in it, therefore the molecular motility is raised and the electric filed is concentrated. In the case of filled specimens with treating silane, the breakdown strength become much higher. Finally, from the analysis of weibull distribution, it was confirmed that to low allowed breakdown probability under 0,.1%, the applied field value needed to be under 21.5MV/cm.