A thermodynamic evaluation has been carried out on the Fe-Si-B system, which is an important system in view of the development of Ni-base filler metals, Ni-base self-fluxing alloys, and core materials for transformers. A regular solution approximation based on the sublattice model was adopted to describe the Gibbs energy for the individual phases in the binary and ternary systems. Thermodynamic parameters for each phase have been evaluated by combining the experimental results obtained by differential scanning calorimetry in the present study with the data from literatures. The set of evaluated parameters in the present study enables reproducible calculations of isothermal section diagrams and vertical section diagrams satisfactorily. Furthermore, the glass forming ability has been also evaluated by combining the thermodynamic quantities obtained from phase diagram calculation with kinetic approach [1]. The agreement between the predicted glass forming ability and the experimental one was very satisfactory.