The dielectric properties of a MWNT/epoxy composite have been studied by impedance spectroscopy. The results were very attractive and showed that the addition of a small amount of MWNTs allows increasing the permittivity of the resin by many orders of magnitude. The insulator-to-conductor transition, which occurred when a few wt% of MWNTs was added to the matrix, is related to a percolation phenomenon with nonuniversal critical exponent. Two types of MWNTs treatment have been compared and the results demonstrate that MWNTs should be used in as-prepared state. The treatment alters their morphology, the aspect ratio can be reduced by a factor of 10, leading to a modification of the shape of the insulator-to-conductor transition curve and a decrease of the global improvement.