The acrylic-based heat-cured soft denture lining material is the most commonly used in relining dentures. This material has poor thermomechanical properties which is a disadvantage. This research aimed to study the effect of the addition of modified metallic nanoparticles on glass transition, modulus of elasticity, and coefficient of thermal expansion and contraction of the acrylic soft liner. Alumina nanoparticles were first modified by a silane coupling agent and then added to a soft denture liner powder in different weight percentages (0, 0.5, 1, and 1.5 wt%) using a probe ultrasonication machine for mixing. 120 samples of acrylic-based soft liner were constructed and divided into four groups G1–G4 (n = 30). Each group was in turn subdivided into 3 subgroups (n = 10) according to the test performed. The mean value, SD, Kruskal-Wallis test, and Dunn’s Multiple Comparison tests were used to analyze the results statistically. Incorporating 0.5% by weight alumina nano-fillers into acrylic-based heat-cured soft denture lining material, increased the glass transition temperature significantly (p ≤ 0.01). Additionally, it significantly reduced the coefficient of thermal expansion and contraction, especially at 30 ºC, compared to the control group. The E-modulus was also reduced, especially at 50 ºC, compared to the control group. According to the reported results, the polymer nanocomposites possess distinctive material properties that distinguish them from unmodified acrylic-base soft denture lining materials. Nanocomposites have more thermal and mechanical stability than unmodified acrylic-base soft denture lining material especially when incorporating 0.5 wt% Al₂O₃.