To improve the toughness of Al2O3 ceramic, xLa2O3/10Nb2O5-(90–x)Al2O3 (x = 2.5-15.0 vol.%) composite ceramics weresynthesized via microwave-sintering. The influences of sintering temperature (Ts) and La2O3 content on phase composition andmechanical property of xLa2O3/10Nb2O5-(90–x)Al2O3 composite ceramics were studied. It is shown in the results that the Tsof La2O3/Nb2O5 doped Al2O3 ceramic was 150 oC lower than that of Al2O3 ceramic. With the increase of x, La0.33NbO3, LaNbO4and LaAl11O18 were formed successively. LaNbO4, columnar-Al2O3 grains and LaAl11O18 with plate-like shape were generatedin-situ during the sintering period. Compared to Al2O3 ceramic, the fracture toughness of xLa2O3/10Nb2O5-(90–x)Al2O3ceramics were at least 70% higher when 5.0 ≤ x ≤ 12.5. 7.5La2O3/10Nb2O5-82.5Al2O3 ceramic exhibited superior mechanicalproperty: Hv = 12.0 GPa, KIC = 6.2 MPa·m1/2 (1,500 oC, 30 min).