In this paper, a fundamental equation for the friction factor of an additional pressure drop due to solid particles is first derived based on the observed dow pattern of solid particles reported in part 1 on a fully developed dow at low fluid velocities in horizontal pipes. Next, an empirical equation for the friction factor is obtained by the substitution of the parameter of the gravitational force and the velocity ratio of particles to fluid determined in part 2 into the foregoing equation. More than 95% of the values calculated by the equation fall within +/-20% of the present experimental values.