This work aims to experimentally investigate the slip velocities of well-mixed particles of two sizes with the liquid now in a vertical column. Tap water is used for the liquid phase. Sieved glass beads having average diameters of 152, 256, 359, 510, and 915 gm are used for the solid phases by pairing particles of different diameters, where the diameter ratio is less than 3.36. It is observed that the slip velocities of smaller and larger particles increase and decrease, respectively, as compared to those in a single-sized particle system of an identical total solid holdup; further, the changes in the slip velocity of a dominant component are smaller than those of an accessory component, and not only are the sizes of the two particles approximately the same, the slip velocities increasingly vary for every component. It is also observed that the differences in the slip velocities of both components from those in the single-sized particle system increase with an increase in the total solid holdup, and that the slip velocities of both particles assume approximately identical values when the total solid holdup exceeds approximately 0.5. The slip velocities of the smaller and larger particles in the two-sized particle system are correlated with the total solid holdup, the ratio of the solid holdup of each component, and ratio of the particle diameter by a novel model. Moreover, we estimate the magnitude of the effects of the drag force, net buoyant forces, and the collision to clarify whether these effects are significant under the current experimental conditions. The results show that the effect of the collision is sufficiently small to he neglected, and that the calculated drag coefficients from our model neglecting the collision effect are in good agreement with that of the experiment.