To investigate the influence of nano montmorillonite(MMT) on the aging resistance and its mechanism of modified asphalt, taking SK90# matrix asphalt and 1%, 3% and 5%（by mass） nano MMT modified asphalt as specimens, three conventional indexes of asphalt and bending beam rheometer(BBR) test were used to measuring the aging resistance of nano MMT modified asphalt. Microstructure evolution of nano MMT modified asphalt before and after aging was observed by atomic force microscopy(AFM). The results show that the changes of penetration, ductility, softening point are improved obviously by nano MMT, the optimum amount is 0%3% in short term aging stage. Stiffness modulus rate Sv of BBR indicates the aging resistance of nano MMT modified asphalt is improved significantly. The microstructure of the asphalt is affected significantly by nano MMT. Found by root mean square(RMS) roughness Sq and 3D image of AFM, nano MMT lamellae and nanocomposite structures formed are the major causes of delaying aging effect.