Alumina (Al2O3), one of the most important ceramics, lacks toughening mechanisms and is sensitive to flaws. Toughening of pure Al2O3 ceramics thus requires emerging mechanisms at the nanoscale, such as grain boundary sliding at finer nanograin sizes. Dense pure and Si-doped Al2O3 nanocrystalline ceramics with grain sizes ranging from 30 to 297 nm were successfully prepared using two-step pressureless sintering. The grain size dependence of fracture toughness in dense pure and Si-doped Al2O3 nanocrystalline ceramics was evaluated by indentation technique. The fracture toughness of pure and Si-doped Al2O3 nanocrystalline ceramics significantly increases with grain size reduction up to 4.45 ± 0.13 and 4.8 ± 0.30 MPa m1/2 at grain sizes of 33 and 30 nm, respectively. This is nearly ∼1.5 times that of the coarse-grained Al2O3 ceramics. The strong grain size dependence of fracture toughness was analyzed by the action of combined grain boundary sliding and migration; and the results simulated from the combined grain boundary sliding and migration model qualitatively match the experimental data. This finding may shed new light on solving the problem of the brittleness of Al2O3 ceramics.