A thermodynamic assessment of the Bi-Mn binary system was conducted using the CALPHAD approach. Consistent thermodynamic descriptions, which agreed well with the selected experimental data, were obtained using a sub-regular solution model for solution phases and a line-compound model for intermetallic compounds. There are several discrepancies between the phase diagram drawn in a previous study and that calculated in this study. The two-phase separation that appeared in the previous phase diagram was not calculated and the calculated liquidus boundaries at around 1000 K changed smoothly compared with that in the previous diagrams. From the viewpoint of thermodynamic consideration, the calculated phase diagram presented in this study is reasonable.