In this study, LaB6-Al2O3 nanocomposite powders were synthesized via ball milling-assisted annealing process starting from La2O3-B2O3-Al powder blends. High-energy ball milling was conducted at various durations (0, 3, 6 and 9 h). Then, the milled powders were annealed at 1200 degrees C for 3 h under Ar atmosphere in order to obtain LaB6 and Al2O3 phases as reaction products. X-ray diffractometry (XRD), scanning electron microscopy/energy-dispersive spectrometry (SEM/EDS) and transmission electron microscopy (TEM) techniques were utilized to carry out microstructural characterization of the powders. No reaction between the reactants was observed in the XRD patterns of the milled powders, indicating that high-energy ball milling did not trigger any chemical reactions even after milling for 9 h. LaAlO3 and LaBO3 phases existed in the annealed powders which were milled for 0, 3 and 6 h. LaBO3 phase was removed after HCl leaching. 9-h milled and annealed powders did not exhibit any undesired phases such as LaAlO3 and LaBO3 after leaching step, and pure nanocrystalline LaB6-Al2O3 composite powders were successfully obtained. TEM analyses revealed that very fine LaB6 particles (similar to 100 nm) were embedded in coarse Al2O3 (similar to 500 nm) particles.