The present study investigates solid–liquid separation behaviors in centrifugal sedimentation for concentrated bidisperse colloidal suspensions containing polymethylmethacrylate (PMMA) particles of two different submicron sizes with equal densities, using an analytical centrifuge. The sedimentation coefficient for describing the settling rate of the interface separating the dispersion from clear liquid was determined from the sedimentation curve drawn based on the measurements of near-infrared light transmission. It was found that the sedimentation coefficient decreased with decreasing porosity and increasing ratio of the volume of small particles to volume of total particles, while it was little influenced by the initial height of suspension and the angular velocity of the rotor. A model was developed for describing the relation between the sedimentation coefficient and porosity in a suspension in the centrifugal sedimentation of a bidisperse suspension. In the low concentration region, the sedimentation coefficient was determined from the settling rate of small particles because small particles settled independently from large particles in the upper zone. In contrast, the sedimentation coefficient in the high concentration region was obtained based on the mean specific surface area size of small and large particles determined using the mixing ratio, since small and large particles settled collectively. In addition, the critical porosity determining the boundary between low and high concentration regions was well described by the model developed. The validity of the model was confirmed by comparing the calculations with experimental data.