Nano-montmorillonite (Nano-MMT)/SBS composite modified asphalts were prepared by melt blending, the aging property of which was detected by rolling thin film oven test (RTFOT) method. The microstructure of Nano-MMT/SBS modified asphalts was characterized by X-ray diffraction (XRD). The results demonstrated that nano-composite structures formed by the intercalation effect of SBS and asphalt into the MMT lamellae, which led to improvement in terms of toughness, strength, and thermal stability. In comparison with SBS modified asphalt, Nano-MMT impact on the properties of the modified asphalt appeared as a decreasing penetration, an increasing softening point, and a decreasing ductility. The anti-deformation ability under high temperature of Nano-MMT/SBS modified asphalt improved significantly. The softening point of the Nano-MMT/SBS modified asphalt decreased after aging, in contrast with that of the base asphalt, which could be attributed to the different rheological properties between the Nano-MMT/SBS modified asphalt and the base asphalt. Compared with that of SBS modified asphalt, the anti-aging property of Nano-MMT/SBS modified asphalt improved dramatically. This indicated that the lamellar structure of montomorillonite in modified asphalt could prevent oxygen from diffusing and permeating in matrix and the thermo-oxidative aging of base asphalt and SBS weakens.