Silica fillers for Epoxy Molding Compounds (EMCs) were modified via plasma polymerization coating of acrylonitile, acrylic acid and dimethyl phosphite with RF plasma (13.56 MHz). The resulting samples were characterized by DSC, FT-IR and contact angle measurements. EMC samples were prepared from silica fillers, biphenyl epoxy resin, phenol novolac and triphenyl phosphine, and cured at 175degreesC for 4 h. Flexural strength of the EMC samples was evaluated in a 3-point bending mode with an Instron 5567 at a crosshead speed of 1 mm/min both at RT and 250degreesC, and failure surfaces were analyzed by SEM. Some samples were exposed to 121degreesC, 2 atm, pressure and 100% RH for 12, 24 and 32 h, and then to 250degreesC for 10 min prior to testing at RT. Plasma polymer coating of silica with acrylonitrile greatly improved the flexural strength of EMC at RT as well as at 250degreesC, followed by acrylic acid and dimethyl phosphite. Exposing EMC samples to 121degreesC, 2 atm pressure and 100% RH for 32 h decreased the flexural strength by 13% when the silica was coated with acrylonitrile plasma polymer, compared to the 21 % decrease in the control sample. Plasma polymer coating of silica also increased the T-g of the EMC, and lowered water absorption and CTE in the rubbery region. Therefore, enhanced properties by plasma polymer coating of silica with acrylonitrile or acrylic acid can be attributed to nitrile or carboxylic acid groups, as confirmed by FT-IR, which can react with epoxy groups in the base resin, as evidenced by DSC analysis.