3D C-f/SiBCN composites were fabricated by an efficient polymer impregnation and pyrolysis (PIP) method using liquid poly(methylvinyl)borosilazanes as precursor. Mechanical properties and microstructure evolution of the prepared 3D C-f/SiBCN composites at elevated temperatures in the range of 1500-1700 degrees C were investigated. As temperature increased from room temperature (371 +/- 31MPa, 31 +/- 2 GPa) to 1500 degrees C (316 +/- 29MPa, 27 +/- 3 GPa), strength and elastic modulus of the composite decreased slightly, which degraded seriously as temperature further increased to 1600 degrees C (92 +/- 15MPa, 12 +/- 2 GPa) and 1700 degrees C (84 +/- 12MPa, 11 +/- 2GPa). To clarify the conversion of failure mechanisms, interfacial shear strength (IFSS) and microstructure evolution of the 3D C-f/SiBCN composites at different temperatures were investigated in detail. It reveals that the declines of the strength and changes of the IFSS of the composites are strongly related to the defects and SiC nano-crystals formed in the composites at elevated temperatures.