Abstract: 3D-C_f/SiC composites were prepared with low-molecular polycarbosilane (PCS) using a precursor infiltration pyrolysis (PIP) process.The average molecular weight of active PCS is 400 (M_n) . The ceramic yield is approximately 70%, and PCS was translated into β-SiC crystal at 1200℃. The samples of 3D-C_f/SiC composites were prepared at three different heating rates.Bending strengthes were respectively 745.2MPa,686.7MPa,and 762.5MPa.SEM photographs of sample fracture surfaces indicate that in the micro-structures there are long fibers pulled out,and the fracture toughness is high.The matrix is close-grained.The research indicates that 3D-C_f/SiC composites with low-molecular polycarbosilane have good mechanical performance,and the low-molecular PCS fits the preparation of 3D-C_f/SiC composites by PIP process.