A magnesiothermic reduction synthesis approach is reported for direct conversion of SiO2/C composite nanostructures to corresponding SiC materials without losing their nanostructure morphologies. Crystalline SiC materials can be obtained by this approach at a temperature as low as 600 degrees C, only approximately half of that applied in the generally used carbothermal reduction and preceramic polymer pyrolysis methods. An ordered hierarchical macro-mesoporous SiC material was synthesized for the first time as a demonstration. This pseudomorphic transformation can be regarded as a general synthesis method for different kinds of SiC nanostructures, and it can also be readily extended to other metal carbide materials as well as TiC.