The hydrodealkylation of 1,2,4-trimethylbenzene (1,2,4-TMB) to benzene, toluene and xylenes (BTX) was investigated on Ni-Mg-Al catalysts prepared by the coprecipitation method. The catalytic performances of these catalysts were considerably influenced by the Mg content of the catalyst. The catalysts were characterized via X-ray diffraction, H-2-temperature-programmed reduction, NH3-temperature-programmed desorption (TPD), CO2-TPD, and Fourier transform infrared spectroscopy. The results demonstrated that the appropriate amount of Mg species significantly affected the structural properties and caused the Ni nanoparticles to become highly dispersed. The higher activity of the catalysts might be ascribed to the homogenous distribution of the Ni nanoparticles, and the synergetic effects between Ni-0, NiAl2O4 and MgAl2O4 were the key factor for obtaining the BTX.