Porous SiC/Si3N4 composites were fabricated by reaction between Si3N4 and C powder in 0.5 MPa nitrogen atmosphere at 1700oC, 1800oC and 1900oC for 0.5h, 1h and 2h, respectively. The characteristics of in situ porous Si3N4/SiC composites were further investigated by X-ray diffraction (XRD) and scanning electronic microscopy (SEM). Results show that the SiC particles are very fine. And the transformation from α-Si3N4 to β-Si3N4 is obvious at 1800oC for 2h and at 1900oC. Otherwise, at 1700oC, the formation of rod-like β-Si3N4 is inhibited. The compressive test was conducted at a cross-head speed of 0.5 mm/min to get the basic mechanical performance of the porous ceramics. The highest value of strength of these composites was obtained when sintered at 1800oC for 1h.