The flame-retardant properties of asphalt for some building applications are very important. In this study, we mainly focused on the influence of organically modified montmorillonite (OMMT) on the flame-retardant and other properties of asphalt in a large content range and explored suitable contents of OMMT for modified asphalts. Modified asphalts with different contents of OMMT from 2 to 15 wt % were prepared by melt blending. The X-ray diffraction results revealed that the intercalated structure was formed in the OMMT-modified asphalt. Rubber processing analysis results indicated the formation of a filler–network structure in the OMMT-modified asphalt. The limiting oxygen index and cone calorimetry results suggested that OMMT could be used as efficient and ideal flame retardants of asphalt. The results also reveal that excess OMMT contents (i.e., >10 wt %) depredated the flame-retardant performance of the modified asphalt. We analyzed the mechanism by taking into account of the features of the modified agent for OMMT. The experimental data showed that the viscosity, softening point, and penetration index of the OMMT-modified asphalt increased with increasing OMMT content, but the ductility decreased slightly when the OMMT content was not beyond 7 wt %. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 10.1002/app.40972.