To tailor a new electromagnetic wave (EMW) absorbing material with lower reflection coefficient (RC) and larger operating frequency band, the CVD Si3N4-SiCN composite ceramics were prepared from SiCl4-NH3-C3H6-H-2-Ar system and then annealed at the temperatures of 1400-1700 degrees C in N-2 atmosphere. Effect of the annealing temperatures on the microstructure, phase composition, permittivity, and microwave-absorbing properties of the ceramic were investigated. Results showed that the CVD Si3N4-SiCN ceramics gradually crystallized into nanosized SiC grains, Si3N4 grains and graphite (T <= 1600 degrees C), and then the grains grew up at T = 1700 degrees C. The permittivity, dielectric loss, and electrical conductivity of as-annealed CVD Si3N4-SiCN ceramics (T <= 1600 degrees C) increased firstly due to the formation of conductivity and polarity network and the increase in nanograin boundary, and then decreased at 1700 degrees C because of the growth of nanograins and the disappearance of nanograin boundary. The minimal RC and effective absorption bandwidth of the as-annealed CVD Si3N4-SiCN ceramic at 1600 degrees C was -41.67 dB at the thickness of 2.55 mm and 3.95 GHz at the thickness of 3.05 mm, respectively, demonstrating that the totally crystallized CVD Si3N4-SiCN ceramic (T = 1600 degrees C) had the superior microwave-absorbing ability.