Abstract: Cracking of 440 MPa 10MnNiCrMoV steel plate was investigated by using analytical equipment such as optical microscopy, scanning electron microscopy and energy dispersive X-ray spectrometer.Results show that the microstructures and hardness adjacent to the cracks were the same with those of the base steel, and a lot of oxides were found on the fracture surface. It is concluded that the defect is herited from a defect in the end of the continuous casting billet and formed in the extension process of plate rolling.