Abstract��Dendritic segregation morphology locating at different areas on the strand of casting billet was observed and analyzed, and segregation microcontent of dendrite and interdendrite were examined in 40MnB steel; Segregation microstructure was researched at different hot rolling reductions; The effect of dendritic distribute and hot rolling microstructure with banded structure was discussed by diffused equation. The results show: Segregation becomes bad gradually from exterior to center in casting billet of 40MnB steel, the element C and Mn segregate in interdendrite, and especially the element Mn is very seriously at the center of casting billet; The difficulty and easiness eliminating banded structure are decided by segregation distribute; It's advantaged to eliminate banded structure that dendritic segregation is lightened in casting billet and hot rolling reduction is increased.