The flame-retardant properties of asphalt for some building applications are very important. This article is mainly focused on the influence of particle size and content of magnesium hydroxide (MH) on the flame-retardant properties of asphalt. The limit oxygen index and cone calorimeter results indicate that as the MH content and mesh number increase, the flame-retardant properties of MH-filled flame-retardant asphalt show a rising trend. But the role of particle size in smoke suppression is not obvious. Several tests confirm that the dispersion of the MH have some influence on the flame-retarding effect of asphalt. The 3000 mesh MH for the preparation of flame-retardant asphalt shows optimal performance. The experimental data show that the softening point of flame-retardant asphalt increases, but the ductility and penetration decrease with increasing MH content. MH affects the asphalt viscosity, but not affects the adhesion of the asphalt to gravel. (c) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013