Objective The aim of this study was to investigate the effects of the ERK5 pathway on bone morphogenetic protein?9(BMP9)?regu?lated osteogenic/odontogenic differentiation of immortalized stern cells from the apical papilla(iSCAP). Methods BMP9 was introduced into the iSCAP by using recombinant adenoviruses,and the P?ERK5 protein expression was measured via western blotting. Then,the osteogenic/odontoblas?tic changes were analyzed by alkaline phosphatase(ALP)staining and Alizarin red staining,and the expression of osteogenic/odontoblast?associat?ed genes,such as Runx2,OCN,OPN,and DMP1 was measured by RT?PCR. Results BMP9 could up?regulate the phosphorylation of ERK5 in iSCAP. After using BIX02189,which was an inhibitor of ERK5,the phosphorylation of ERK5;the activity of ALP;the expression of Runx2, OCN,OPN,and DMP1;and the calcium deposition were all significantly inhibited. Conclusion The ERK5 signaling pathway plays an important role in BMP9?regulated osteogenic/odontogenic differentiation of iSCAP.