Abstract: The article studied the microstructures of TiC-50Nb steel-bonded cemented carbide quenched at 1120℃ and annealed at 560℃ by means of TEM. The results showed that the microstructure of the TiC-50Nb steel-bonded cemented carbide through above mentioned treatment, especially the microstructures of their steel matrix had an important influences on the macro-properties. The carbides precipitated from steel matrix were the main causes of producing secondary hardening phenomenon.