This paper investigated the effects of MnS inclusions on inducing pitting in steel. Three different cooling methods were used to control the morphology, size, and distribution of MnS inclusions. AC (alternating current) impedance spectroscopy experiments and potentiodynamic polarization experiments were carried out for different sizes of MnS inclusions. The corrosion immersion experiment was conducted to study the pitting corrosion process in different morphologies of MnS inclusions. The experiment results showed that pitting corrosion occurred around MnS inclusions and pitting was induced along the junction of MnS inclusions and their surrounding steel matrix. Also, the sensitivity of inducing-pitting of MnS inclusions was changed with the change of the average size of MnS in steel and it had a critical size of MnS. The deeper the MnS inclusions were buried in steel matrix, the more difficult it was for it to dissolve and fall off. Herein, it would cause greater damage.