Nano-silica particles were employed for enhancement of epoxy vacuum pressure impregnating (V.P.I.) resin. The influences of nano-silica particles on the curing reaction, glass transition temperatures, dielectric behavior, and thermomechanical performances were investigated. The activation energy (E) for the epoxy curing reaction was calculated according to Kissinger, Ozawa, and Fried man-Reich-Lev methods. The glass transition temperatures were determined by means of differential scanning calorimetry, dynamic mechanical analysis, dc conduction, and ac dielectric loss analysis. Relationships between the glass transformation behaviors, the thermomechanical performances, and the dielectric behaviors were discussed. The influences of nano-silica particles on the mechanical properties were also discussed in terms of non-notched charpy impact strength and flexural strength. The morphologies were studied by means of SEM and TEM. The results indicated that nano-silica particles could effectively increase both the toughness and strength of epoxy resin at low loadings (no more than 3 wt %) when nano-silica particles could be well dispersed in epoxy matrix without any great aggregations. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 111: 917-927, 2009