This study presents a numerical model to quantitatively predict a cleavage fracture toughness of ferrite-cementite steel without
any adjustable parameters. The model is constructed in framework of multiscale analysis. The macroscopic analysis is an elastoplastic
finite element analysis. A microscopic analysis is based on the Monte Carlo method considering microscopic fracture
process of the three stages; Stage-I: formation of fracture origin by cementite cracking, Stage-II: propagation of the cementite
crack into ferrite matrix and formation of a cleavage crack, and Stage-III: propagation of the cleavage crack across ferrite grain
boundary. The predicted results of fracture toughness show good agreement with the experimental results including their scatter.
Influences of microstructures on fracture toughness can be estimated by parameter studies. According to the results, the
validation and the effectiveness of the proposed model are found out.