Electrically conducting adhesive films containing silver nanosized particles (AgNPs) were prepared using siloxane polyimide resin (SPI), and their physical properties were investigated. AgNPs with diameters of ca. 6 nm were prepared using oleylamine as a stabilizer. A mixture of the AgNPs and SPI was vigorously stirred, and cast on a polyethylene terephthalate support. Films were then fabricated using a doctor blade method. The heat resistance of the SPI film containing 60 wt% AgNPs was 35 °C higher than that of the SPI film without AgNPs. In addition, epoxy composite SPIAg (Epo-SPIAg) films containing a mixture of AgNPs and micro-sized silver flakes were prepared. Inclusion of AgNPs in these films strengthened their adhesion to silicon or bismaleimide triazine resin substrates. The glass transition temperature of the Epo-SPIAg films containing AgNPs was slightly higher than that of the film containing only micro-sized silver flakes.