Abstract: In-situ observation of the behavior of MnS inclusions in non-quenched and tempered steel with a sulfur mass fraction of 0.065% was performed during tensile tests by using a laser confocal scanning microscope (LCSM) to investigate the effects of the morphology and distribution of MnS inclusions on the tensile property anisotropy of the steel. The result shows that there exist a large number of rod-like MnS inclusions in the forged steel, which have different orientations with the load during transverse and longitudinal tensile tests. The rod-like MnS inclusions separate much easier from the steel base during transverse tensile tests to form initial cracks, and then the cracks spread along the MnS inclusions. In this way, the cracks grow up and lead to the fracture of the steel. In contrast, the MnS inclusions have little effects on the longitudinal tensile properties because the length direction of MnS is parallel to the tensile force. MnS inclusion aggregation promotes the assembling and growth of single cracks, thus accelerating the fracture of the steel during tensile tests.