Aiming at developing novel microwave-transparent ceramics with low dielectric loss,high thermal conductivity and high strength,Si3N4-AlN (30％,mass fraction) composite ceramics with La2O3 as sintering additive were prepared by hot-pressing at 1 800 ℃ and subsequently annealed at 1 450 ℃ and 1 850 ℃ for 2 h and 4 h,respectively.The materials were characterízed by XRD and SEM.The effect of annealing process on the phase composition,sintering performance,microstructure,bending strength,dielectric loss and thermal conductivity of the materials was investigated.The results showed that both annealing at 1 850 ℃ and 1 450 ℃ promoted the phase transformation of α-Si3N4 to β-Si3N4.After annealing at 1 850 ℃,grain growth to a certain extent occurred in the materials.Especially,the elongated,β-Si3N4 grains showed a slight increase in diameter from 0.2 μm to 0.6 μm approximately and a decrease in aspect ratio.As a result,as the annealing time increased to 4 h,the bending strength declined from 456 MPa to 390 MPa,whereas the dielectric loss decreased to 2.15×10-3 and the thermal conductivity increased to 16.3 W/(m·K) gradually.When annealed at 1450 ℃,increasing the annealing time to 4 h significantly promoted the crystallization of glassy phase to La2Si6N8O3 phase in the materials,which led to the increase in bending strength to 619 MPa and thermal conductivity to 15.9 W/(m·K),respectively,and simultaneously the decrease in dielectric loss to 1.53×10-3.