This study reports the effects of some milling parameters (duration, ball-to-powder weight ratio (BPR), process control agent (PCA)) on the fabrication of nanocrystalline LaB6 powders via a combined process of mechanochemical synthesis (MCS) and acid leaching, starting from La2O3-B2O3-Mg powder blends. MCS was carried out in a high-energy ball mill (SPEX (TM) 8000D Mixer/Mill, at 1200 rpm) using hardened steel vial and balls under Ar atmosphere. As-synthesized products were leached out with 4 M HCl from unwanted MgO phase and probable Fe impurities. When a BPR of 18:1 was used, 5 h of MCS and leaching yielded LaB6 powders with an average particle size of 100 nm, containing a very small amount of FeB49 phase. Pure LaB6 powders were obtained with an average particle size of 53 nm after 12 h of MCS of the 0.5 wt% PCA added La2O3-B2O3-Mg powder blends with a BPR of 10:1 and after leaching.