Alumina-based die-attach and encapsulation for high-temperature (300–500 °C) electronic packaging were investigated. The alumina paste material comprises aluminum dihydric phosphate as a binder and alumina powder as a filler with embedded nano-aluminum nitride and nanosilica powders to promote its curing process, reduce its curing tension, and increase its bond shear strength. The chip-to-substrate bond strength was enhanced and met the MIL-STD-883 2019.9 requirements for die-attach assembly. Its encapsulation property was improved with fewer cracks compared to similar commercial ceramic encapsulants. The die-attach material and encapsulation properties tested at 500 °C showed no defect or additional cracks. Thermal aging and thermal cycling were carried out on the samples. X-ray photo-electron spectroscopy (XPS) analysis revealed a higher oxygen bonding percentage for the 10% nanosilica ceramic sample than the samples with no nanosilica. XRD peak broadening is largest for the 10% nanosilica ceramic, which indicated smaller crystallite sizes. The smaller crystallite size for the 10% nanosilica sample introduces a larger microstrain to the alumina crystal structure. FTIR revealed the presence of alumina-silicate bonds on these samples with the largest amount present in the 10% nanosilica samples. Si-O and Al-O bonds were observed from FTIR on nanosilica samples especially the higher than 10% nanosilica samples. SEM and energy dispersive X-ray (EDX) results showed a uniform bond line for the 10% sample and uniform material distribution.