Abstract: The Ce and P cosegregation on the grain boundaries in a-iron was studied by Auger Electron Syectroscopy (AES) and computer modelling method. The result shows that in case of low P content in the alloys (P<0.07wt%), Ce reduces the grain boundary segregation of P, but in the high P alloys (P>0.10wt%), Ce enhances the segregation of P. The computer modelling of this phenomenon indicates that the segregation structure of Ce and P on the grain boundary varies with the change of P content on the boundary. The main reason of the change of the interaction between Ce and P on the grain boundary is that the lowest energy structure of Ce-P cosegregation varies.