Abstract: The total oxygen content(T. O),non-metallic inclusions and the relationship between them in pipeline steel,gear steel,tire cord steel and Q235 steel,which were taken from industrial products,were systematically investigated by ASPEX automatic scanning electron microscope(SEM),which can scan large area metallographic specimen,and thermodynamic software Thermo-Calc was used to calculate and validate. The obtained results indicate that non-metallic inclusions are greatly influenced by the production process of steel. With the inclusion size increasing,the amount of inclusions in this size range decreases. Both quantity and area of inclusions increase sharply when T. O increases from less than 0.002% to 0.01%. When T. O is lower than 0.002%,it only shows good positive relationship with the area of all inclusions per square mm steel samples. T. O indicates the area of inclusions in steel more accurately than the quantity of inclusions. Large sized inclusions appear accidentally. The experimental results agree very well with thermodynamic calculations and the content of T. O,S in steel.