Based on thermodynamics calculation,the results of the formation temperature of MnS inclusions of non-quenched and tempered steel during heating process were discussed. It is shown that while the solid fraction is 0.9,MnS inclusions began to precipitate in the final stage of solidification.The solidification process of 49MnVS3 non-quenched and tempered steel during heating has been observed in situ using a confocal scanning laser microscope (CSLM),which agrees well with the thermodynamics calculation. MnS particles were coarsening during heating process,which would reduce the pinning effect on the austenite grain boundaries and bring about the sudden growth of some austenite grains in this stage.