Numerous studies have been conducted to clarify the relationship between microscopic factors and the viscosity of particle suspensions. Segre and Silberberg [1] observed that a single rigid particle in pipe flows in which inertial forces are considered migrated to an equilibrium position with its center around 0.6R, where R is the pipe radius. Fukui and Kawaguchi [2] analyzed suspension flow with randomly distributed particles and demonstrated that the relative viscosity was strongly affected by the Reynolds number. This is because the microscopic particle arrangement in the channel changes in response to the inertial forces acting on the particles. To validate these reports, comparisons with experiments are indispensable. Sample suspensions used in the experiments contain slight particle size differences and the inertial forces acting on an individual particle is accordingly different, which may be expected to affect microscopic particle behavior and relative viscosity. Therefore, we performed a suspension flow simulation with size-dispersed particles to investigate the effect of dispersion on particle arrangement and relative viscosity. The governing equations were the regularized lattice Boltzmann method, and the modeling scheme of a circular rigid particle was the virtual flux method. Particle size differences were defined by a Gaussian function, with the standard deviation determined based on the CV values (CV = 20%). The particle Reynolds number is Rep = 0.058 ~ 0.314 corresponding to flow conditions with exerting sufficient inertial force on the particles. A comparison of the ideal model without dispersion and the model with dispersion (CV = 20%) confirmed that the particle arrangement and relative viscosity were unaffected in the present condition. In the future, we will conduct an exhaustive survey to confirm the effect of particle dispersion under a broader range of flow conditions with varying particle Reynolds number.