In the present study, electrochemical reaction mechanisms of La0.6Sr0.4CoO3 (LSC)-Gd0.1Ce0.9O1.95 (GDC) composite cathodes were investigated by a Lattice Boltzmann Method (LBM) based on 3D reconstructed cathode microstructures with different volume ratios. In the simulation, the exchange current density for the surface reaction was fitted using an experimental data of pure LSC cathode. On the other hand, the exchange current density for the TPB reaction was fitted using composite cathodes. Overpotentials and potential distributions were quantified. Then, the results were compared with those of LSCF-GDC composites reported in our previous work (Y.T. Kim, Z. Jiao, N. Shikazono, J. Power Sources, 342, pp. 787-795, 2017). LSC surface area density was 1.4 μm²/μm³, which was smaller than the LSCF cathode. Compared with the LSCF-GDC composite cathode, it is considered that TPB reaction is more contributed for the LSC-GDC composite cathodes.