The local relative velocity model proposed by Tomiyama, et al. was applied to the estimation of solid volume fraction in solid-liquid two-phase pipe flows with a wide range of the diameter ratio of particle diameter to pipe diameter. The diameter ratio ranged from 0.022 6 to 0.818. No estimation methods proponed so far can cover this diameter ratio range. The local relative velocity model takes into account the effects of the distributions of local volume fractions and local velocities in a cross-sectional area of a pipe, being similar to the drift-flux model. An empirical correlation of the terminal velocity of a single solid particle falling in a liquid-filled pipe was applied to the phase-averaged “local relative velocity”, and a simple empirical correlation of another parameter in the local relative velocity model was deduced from measured solid volume fractions and phase-averaged velocities. Substituting these two correlations into the local relative velocity model, we developed a new simple correlation of solid volume fraction. This correlation was confirmed to yield good estimations of solid volume fractions both qualitatively and quantitatively.