Structural materials for generation-IV nuclear reactors must withstand extremely harsh environments involving high temperatures, corrosive coolants, and severe radiation, which render conventional nuclear structural materials ineffective. Owing to its excellent high-temperature stability, remarkable mechanical properties, and outstanding resistance to irradiation damage, oxide dispersion-strengthened (ODS) steel is considered a promising candidate for advanced nuclear power systems. The addition of Al and Si to 14Cr-ODS steel can improve its corrosion resistance. However, research on the long-term high-temperature stability of 14Cr-ODS steel with the addition of Al and Si, which is critical for ensuring reactor operational reliability, remains inadequate. In this study, the microstructural evolution, hardness and tensile - properties of two 14Cr-ODS steels containing 4%Al + 0.6%Si and 4%Al + 1.5%Si (designated as 0.6Si-ODS and 1.5Si-ODS, respectively) were investigated after long-term aging at 700 °C for 10000 h. The results showed that the addition of 0.6%–1.5% Si exerted no notable influence on the microstructure and mechanical properties of the steels before and after aging. Nanoparticles uniformly dispersed were identified as YAlO₃, Y₄Zr₃O₁₂, Y₂(Zr_0.6Ti_0.4)₂O₇, and Y₂Si₂O₇ by TEM. In addition, a small amount of 200 nm Si-rich oxide particles were found in 1.5Si-ODS steel. The nanoparticles with an average size of (6.3 ± 0.8) nm were observed in 1.5Si-ODS, whose number density reached 2.3 × 10²³ m⁻³. After aging at 700 °C for 10000 h, the hardness and tensile properties of both the steels at room temperature and 700 °C remained essentially unchanged. The ultimate tensile strength of 1.5Si-ODS decreased only slightly from 214 MPa to 210 MPa, and the elongation decreased from 37.5% to 36.8% at 700 °C, which is markedly higher than that of most reported Si-containing ODS steels. Microstructural observations confirmed that the grain size, nanoparticle size, and number density of nanoparticles exhibited excellent stability. These stable nanoparticles continuously pinned grain boundaries, effectively hindering the movement of dislocations and recovery of grain or subgrain structures, ensuring the stability of mechanical properties.