The interfacial electrical tree (IET) between the packaging insulation and the ceramic substrate is easily initiated by a high electric field around the triple point in the power modules, and the IET resistance is not considered in the current packaging insulation design. This article proposes a low-content POSS-grafted silicone elastomer nanocomposite to hinder the IET growth, and its electrical, thermal, and mechanical properties are tested. The IET patterns and the accompanying partial discharge (PD) characteristics are also investigated in the lateral and vertical packaging modules at low and high temperatures. The IET in the packaging modules appears at a much lower voltage than their breakdown voltage and takes longer until the ultimate breakdown of the modules. Besides, the IET is initiated at a lower voltage and grows faster at higher temperatures. The grafted nanocomposite packaging insulation inhibits the IET growth and decreases the IET breakdown probability by its reduced charge carrier mobility and enhanced molecular interaction. The mechanisms of inhibited charge transport on the enhanced electrical properties and hindered IET growth are revealed. Combined with the quantum chemical calculations, bipolar charge transport, and IET phase-field simulations, the design criteria and procedure for high-temperature IET-resistant packaging insulation are proposed.