Abstract:The 300M steel bars with diameter of 450 mm were inspected by ultrasonic method. 2.5 MHz soft probe was used to ensure ultrasonic sensitivity and reduce the noise level. Macro- and micro-observations as well as electrical diffraction spectrum analysis were carried out to inspect interior defects of a material. The results showed that inclusions found by ultrasonic detection were consisted of Al2O3,MgO and CaO. The largest size of the inclusions was over 100 μm. These inclusions were imported by melting process, and they were detrimental to properties of the material.