Non-metallic inclusions have become a matter of concern for several properties of steel. The effective evaluation of inclusions in steel would provide critical information to evaluate the steel quality. In this study, an evaluation method based on X-ray Micro-Computed Tomography (Micro-CT) experiments was proposed. First, Micro-CT experiments were carried out to characterize the inclusions. Then, the characterizations of the inclusions were collected based on the 2D slices and their 3D features. The extreme values were evaluated by statistics of extreme values method with these data. The evaluation results were compared with those obtained by international standard ASTM E2283-08. The results demonstrated that the evaluation method based on inclusion sizes collected from 3D inclusion information provided a stable result. It means that the extreme value evaluation method with the 3D data based on Micro-CT was an effective method.