SiC-Al2O3 composite powder was prepared by sol-gel and carbothermal reduction method. The powder synthesized was characterized by X-ray diffraction(XRD) and scanning electron microscopy(SEM) to confirm the phase formation, and the thermodynamic analysis was performed systematically. Moreover, the variation of its microwave permittivity with different atomic ratio of Al/Si was investigated in the frequency range of 8.2-12.4 GHz. The results show that, the powder obtained consists of spherical particles of 300-400 nm in diameter, which are composed of SiC and Al2O3 microcrystal with the grain size of approximately 45 nm. The results of XRD accord with those of the thermodynamic analysis. It is impossible for Al atoms to dissolve in the lattice of SiC during the carbothermal reduction process. Along with the increase of atomic ratio of Al/Si in the xerogel, the amount of Al2O3 in the powder synthesized increases, which reduces bothε’, the real part of complex permittivity, and tgδ(ε“/ε’), the dissipation factor, whereε” is the imaginary part of complex permittivity. SiC-Al2O3 composite powder was prepared by sol-gel and carbothermal reduction method. The powder was characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) to confirm the phase formation, and the thermodynamic analysis was performed systematically. The results of that microwave permittivity with different atomic ratio of Al / Si was investigated in the frequency range of 8.2-12.4 GHz. The results show that the powder obtained consists of spherical particles of 300-400 nm in diameter, which are composed of SiC and Al2O3 microcrystal with the grain size of approximately 45 nm. The results of XRD accord with those of the thermodynamic analysis. It is impossible for Al atoms to dissolve in the lattice of SiC during the carbothermal reduction process. Along with the increase of atomic ratio of Al / Si in the xerogel, the amount of Al 2 O 3 in the powder grown increases, which reduces both ε ’, the real part of complex permittivity, and tgδ (ε “ ε ’), the dissipation factor, whereε ”is the imaginary part of complex permittivity.