Resorcinol-based phthalonitrile (R-CN)/nano-aluminum oxide (Al2O3) nanocomposites were prepared via a two-step approach. Firstly, Al2O3 was functionalized with nitrile groups on the surface of Al2O3 nanoparticles, which was confirmed by Fourier transform infrared spectroscopy, thermogravimetric analysis, and scanning electron microscopy (SEM). The effect of nano-Al2O3 particles on thermomechanical and flexural properties has been evaluated for different weight ratios ranging between 0% and 5%. Compared with pure nano-Al2O3, nitrile-functionalized Al2O3 (CN-Al2O3) particles showed a more significant enhancement effect on the properties of R-CN resin. The storage modulus of nanocomposite with 5 wt% CN-Al2O3 reaches 2679 MPa at 25°C, which is much higher than that of the pure R-CN resin. For 3 wt% CN-Al2O3-reinforced R-CN composites, it showed an increase of 54.84% in flexural strength and 21.48% in flexural modulus. SEM was employed to explore the fracture surface of composites. Micrographs of fracture surface analysis confirmed that the toughness of R-CN resin can be improved significantly by incorporating CN-Al2O3 nanoparticles.