Al 2 O 3 ceramics doped with 15%Nb 2 O 5 and 7.5%La 2 O 3 (volume percent) were prepared by microwave sintering at 1450~1550℃ for 30min. The effect of sintering temperature on the microstructure, especially Al 2 O 3 columnar grains, and the fracture toughness were investigated. The results show that the phases of sintered samples were α-Al 2 O 3 , LaNbO 4 and Nb 2 O 5 , without La 2 O 3 . During the sintering period, the in-situ reacted LaNbO 4 , and the remained Nb 2 O 5 after reacting, changed to liquid which promoted the growth of Al 2 O 3 columnar grains and sintering densification. With the increase of sintering temperature, more and more columnar Al 2 O 3 grains grew instead of equiaxial grains, and the columnar grains length increased from 8μm to 22μm. The average ratio of length to diameter of columnar grains increased firstly and reached to the maximum of 5:1 at 1500℃, then keep unchanged. The fracture toughness increased with the increase of sintering temperature, from 4.83 to 6.48 MPa·m 1/2 .