Abstract: Nanoparticles have been widely used in polymer gel systems in recent years to improve gelation performance under high-temperature reservoir conditions. However, different types of nanoparticles have different effects on their gelation performance, which has been little researched. In this study, the high-temperature gelation performance, chemical structure, and microstructure of polymer gels prepared from two nanomaterials (i.e., nano-SiO2 and nano-TiO2) were measured. The conventional HPAM/PEI polymer gel system was employed as the control sample. Results showed that the addition of nano-TiO2 could significantly enhance the gel strength of HPAM/PEI gel at 80 °C. The gel strength of the enhanced HPAM/PEI gel with 0.1 wt% nano-TiO2 could reach grade I. The system also had excellent high-temperature stability at 150 °C. The enhanced HPAM/PEI gel with 0.02 wt% nano-TiO2 reached the maximum gel strength at 150 °C with a storage modulus (G′) of 15 Pa, which can meet the need for efficient plugging. However, the nano-SiO2 enhanced HPAM/PEI polymer gel system showed weaker gel strength than that with nano-TiO2 at both 80 and 150 °C with G′ lower than 5 Pa. Microstructures showed that the nano-TiO2 enhanced HPAM/PEI gel had denser three-dimensional (3D) mesh structures, which makes the nano-TiO2 enhanced HPAM/PEI gel more firmly bound to water. The FT-IR results also confirmed that the chemical structure of the nano-TiO2 enhanced HPAM/PEI gel was more thermally stable than nano-SiO2 since there was a large amount of –OH groups on the structure surface. Therefore, nano-TiO2 was more suitable as the reinforcing material for HPAM/PEI gels for high-temperature petroleum reservoir conformance improvement.