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.