The growth of Al–Mg–B thin films on SiO2/Si(100) substrates was performed by nanosecond pulsed laser deposition at three different substrate temperatures (300 K, 573 K, and 873 K). The as-deposited films were then annealed at 1173 K or 1273 K for 2 h. X-ray photoelectron spectroscopy,x-ray diffraction(XRD), and atomic force microscope were employed to investigate the effects of processing conditions on the composition, microstructure evolution, and surface morphology of the Al–Mg–B films. The substrate temperatures were found to affect the composition of as-deposited films in that the Mg content decreases and C content increases at higher substrate temperatures, in particular for the 873 K-deposited film.XRD results show that the as-deposited films were amorphous, and this structure may be stable up to 1173 K. Annealing at 1273 K was found to fully crystallize the room temperature and 573 K-deposited Al–Mg–B films with the formation of the polycrystalline orthorhombic AlMgB14 phase, accompanied by the development of a pronounced (011) preferred orientation. Nevertheless, high C incorporation in the 873 K-deposited Al–Mg–B film inhibits the crystallization and the amorphous structure remains stable even during 1273 K annealing. The presence of Si in the room-temperature-deposited 1273 K-annealed film due to the interdiffusion between the substrate and film leads to the formation of an additional tetragonal α-FeSi2 phase, which is thought to cause the surface cracking and microstructural instability observed in this film.