Two-fluid Eulerian method is widely used in computer simulations of gas-solid systems such as fluidized beds. The models for describing drag forces play key roles in the reliability of simulation results. The solid-solid drag model is calculated by the formulas such as Syamlal-O'Brian's model [Syamlal, M. The particle-particle drag term in a multipardcle model of fluidization. Topical report, DOE/MC/21353-2373, NTIS/DE87006500, National Technical Information Service, Springfield, VA. 1987]. However, the performance of the model has not been examined in the mixtures of solid phases with a large size ratio between the particles of the phases, which is found in fluidized beds containing biomass particles (big particles) and the neutral phase particles (small particles) such as sand. The present study compares the results of Eulerian simulations using the Syamlal-O'Brian's formula with the results of Lagrangian simulations performed by the discrete element method (DEM) in the bidisperse mixtures of the size ratio of 10. Results suggest a significant difference between the Eulerian model and the DEM, which is basically due to the inhomogeneities triggered by the existence of large particles. The current methodology can be used to present correction factors for the solid-solid drag forces in Eulerian simulations for the bidisperse mixtures of large size ratios. (C) 2014 Elsevier B.V. All tights reserved.