It is well known the settling behavior of fine particles in liquid changes at a certain particle concentration. Particles, which settle independently, coagulate each other and make a clear interface over a certain concentration. The particle concentration affects the settling behavior as the gap between particles, which is a very important factor of particle-particle interaction. In this paper the average particle interval, distance to the closest particle and contact particle number on a specified particle is discussed geometrically in cases of a mono-size and multi-size particle system. And the collision free path of multi-size particles settling under the gravity is also discussed. The calculated results can interpret experimental results.